dying fish consistent problem

I am going to assume you have good test kits. Such as the kind that requires you to add drops of reagent to a glass test tube and that your test results are ok.
So with your years of experience, I will go directly to what I suspect first.
I think you may have a trace or small amount of electricity getting into the water either from your pumps or maybe the UV or by induced current from your lights being too close to the water surface. If you are familiar how a transformer works, then it about the same. If a core with many wraps of wire is placed next to another core with more or less wraps and an current is applied to one of the core wires, then the other core is affected by the electromagnetic field and creates a lower or higher voltage. Now this is a possibility in your case.
If my suspicions are correct, then I would start first with the UV and stop using it. If you have a voltmeter, set it for 50 volts or less and attach a grounding wire to the screw of a electrical receptical. Place one voltmeter lead into the water and the other on the grounding wire (keep your fingers away from the metal part of the leads) and see if you get a reading on the AC and DC scales. write down the voltage no matter how small. If you get a reading, then you need to test all equipment that uses electricity by shutting them off, one at a time to find out which one gives the highest reading. Don't forget the heater.
To date, I do not know of any research performed on how much induced current it takes to affect the health of aquarium inhabitants. Many years ago, there was a discussion on another forum about it.
I tested my water back then and got a reading and my tank was grounded which I suspect made it worse because it created a path for the electricity to flow.

The other suspicion which is much easier to correct is how you vacumn your gravel. Some fishkeepers say that if you push the pickup tube below the surface, it disturbs the nitrifying bacteria and pulls it away from the gravel and an ammonia spike can occur. By the time you test the water, the spike has been corrected but the effects on the fish are apparant. I am not sure that theory is correct but to cover myself, I use a product that adds nitrifying bacteria when I vacumn every two months.

Here is an interestion read on the subject of stray current in the aquarium and the following content is made available under Creative Commons Attribution - Share Alike 3.0 Unported License.

Stray current


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Stray current refers to unintentional electricity flowing through the aquarium water. Malfunctioning and deteriorating equipment are the usual sources and can represent a danger to the aquarist and livestock. Stray current is closely related to stray voltage - a voltage potential between the aquarium water and ground.
Contents

[hide]

• 1 Stray current vs stray voltage
• 2 Symptoms
• 3 Sources
  • 3.1 Leaks
  • 3.2 Magnetic Induction
  • 3.3 Static electricity
• 4 Identifying
• 5 Recommendations
• 6 Links
• 7 References

Stray current vs stray voltage

In typical household circuits, current is far more dangerous than voltage.
Stray voltage is an electrical potential difference between the water and earth ground. A voltage difference does not indicate any electricity is actually flowing through the water. It indicates the possibility of current to flow if a suitably low resistance path between the water and ground is introduced. So stray voltage on its own is not directly harmful. It is common to have some stray voltage as pumps can induce a voltage potential with their rotating magnets.
Stray current refers to flowing electricity. It should be considered dangerous at even the milivolt range. When wet, the body's electrical resistance is dramatically reduced allowing more current to flow. At the 100ma range, current can cause ventricular fibrillation (uneven, uncoordinated pumping of the heart), muscular contraction, nerve damage, death is likely [1]. A properly functioning tank should not exhibit any measurable stray current (< 1mA). Stray current may or may not trip a GFCI receptacle depending on the path it takes.
Symptoms

• shock and/or tingling when touching the water, in particular on cuts
• shock and/or tingling when touching the water and a metal lighting fixture
• GFCI outlet tripping
• odd behaviour from fish or coral
• possible cause of Head and Lateral Line Erosion (HLLE) disease
• erroneous readings from pH probes

Sources

Leaks

An energized conductor is exposed to the aquarium water. This usually occurs when equipment is worn or damaged. For example, an electrical cord where the insulation is cracked or worn away. Other examples includes cracked housings on pumps and heaters. This is dangerous as the full current of the outlet may flow if the aquarist touches the water and a ground (like a grounded metal fixture). In rare cases, under the right circumstances (Murphy's Law), when there are multiple electrical cords/components in the water (which is the case with most of our systems), there is the possibility of current flowing from Line to Neutral or Ground from one cord to another. This current will flow from one component to the other, through the water & may not necessarily be measurable with a meter to ground. It will most probably affect your livestock though.
Magnetic Induction

Moving magnetic fields can induce a voltage and current in the aquarium water (or any conductor). Pumps and powerheads often have spinning magnets and may generate up to 50v of voltage but only very small amount of current. Flourescent bulbs (PC, VHO, T5) will also induce voltage and current. The current induced by these sources is small, in the nanovolt range [2].
Static electricity

This may possibly be a small source of stray voltage. The large conductive surface film will likely dissipate any charge.
Identifying

• Visual Inspection
  • Remove all equipment (perhaps as part of regular cleaning and maintenance)
  • Inspect cords for worn, chewed or damaged sections
  • Inspect where the cord enters the equipment and connects to the electrical plug. Is it secure and undamaged
  • Look for cracked housings on pumps
  • Look for water inside glass heaters

WARNING - This procedure deals with mains power and is potentially fatal. If in doubt, contact an electrician. Consider performing a visual inspection as decribed above instead.

• Measuring with multimeter
  • Ensure all equipment is connected to a GFCI outlet. While this is not required for to measure current it will significantly reduce the risk of harm and/or death if a mistake is made.
  • Ensure you have a clear, dry area to work. Reduce the risk of accidentially touching anything in particular the water or ground.
  • Turn on all electrical equipment in contact with the water
  • Configure the multimeter to measure current (mA)
  • Some multimeters have multiple probe sockets, ensure your probes are in the correct sockets for measuring current
  • For older multimeters without auto-ranging: select the highest range
  • Place black probe in the third (ground) prong of the GFCI outlet (or an attached three prong extension cord)
  • Place red probe in the water
  • For older multimeters, if the meter reads 0 select the next lowest range and repeat until the meter reads non-zero or you select the lowest range.
  • If the multimeter reads 0ma, you are done, there is no stray current
  • Disconnect equipment one by one (don't forget float switches)
  • When the current reading drops, the last piece of equipment you disconnected is a source of stray current
  • Immediately replace all equipment contributing to stray current

Recommendations

• Connect all equipment to one or more GFCI protected outlets
• Periodically inspect equipment using the Visual Inspection procedure above
• An aquarium should have no detectable stray current (detectable by a common multimeter). Replace any equipment contributing stray current.
• Minimize contact between equipment and the water
  • Keep the control dial and cord on heaters above the water line
  • Keep electrical cords out of the water
  • Use external (non-submerged) pumps
• Use drip loops
• Consider low voltage DC pumps (EcoTech, Tunze, Koralia?) and LED lighting to reduce risk
• Keep high voltage AC equipment as far away from the water as practical
• Consider using moisture resistant electrical receptacles such as those used outdoors
• Consider installing a ground probe (only if GFCI is present). There is debate as to the benefits of a ground probe.
  • At current levels above the GFCI trip current, the ground probe will trip preventing the current from flowing through someone contacting the water later.
  • At current levels below the GFCI trip current, the ground probe will allow current to flow where it would not have before potentially affecting livestock.
  • Do not use a ground probe to solve a problem with low levels of stray current, instead identify and replace the source.

wow, really cool theories, I will get on these tests tomorrow.
I don't have GFCI.
I've only replaced one of the filters.
so, a lot of the equipment is original.
I have an inline heater, a powerhead at the bottom for more water movement so there is a cord submerged, and compact flourescent lighting close to the water.
The UV I added about a year and a half after starting the tank.
Nitrite, can't remember if I ever checked the tap water for it, but I have checked
after a water change in the past.
I don't have trouble with my other tanks though.
Lost interest in testing, since always checks out good.
have several different brands of each test to make sure no issue with test quality.
I have not checked for ammonia after gravel vac as far as I remember, always tested after water was replaced and before cleaning.
The idea about current is something I haven't thought about.
Very interested to see the result.
Thank you very very much!
Terri

the suspense was killing me so I checked for current just now,
Underwater powerhead and nightlight LEDs do not put out current.
Every other single device puts current into the water.
With everything on the reading is .027 using a digital multimeter set at 3
I don't know much about electricity.
filter .003
filter .004 both running becomes .005
.006 UV
.008 lights
.030 inline heater, on or off
all items plugged in and running .027
So, scrap the heater and use the UV for heating for now.
Will have to see how the other tanks test out.
Thank you so much!
Terri

Are these readings ac or dc? Some of the equipement we use operates either on ac or dc. My pH meter has an adapter that converts ac to dc. My led's have an ac imput but operate on dc. Aquarium pumps and powerheads are ac operated unless they have an electrical adapter. So some of your readings may be in error. The author states that current produced by Flourescents, T5, Pc and VHO produced currents in the nano range and we need not worry about that. He suggests that we should not get any readings above 0.001 amp or 1 milleamp. But if all the equipement is reading higher, then I would suspect its normal unless the equipement is old and possibly deteriorated and the authors suggestion may be questionable. We need to find out more by everyone testing their water for current as outlined in the authors directions. Then we will know what is normal; a little current or no current. I think this is a subject that the manufacturers do not want to touch.
In the end, having a gfci to protect you is a good idea, but using a grounding rod in your aquarium creates a path for current to flow when it occurs and is not a good idea for the tank inhabitants.
The differance between the two is that the ground rod may have a current flow that is less then what a gfci can detect. So the question is: how many milleamps is that and is it enough to affect the tank inhabitants?

I have a complicated digital multimeter and a tiny analog one and a cheap new digital that gives weird high readings, I don't trust it.
I understand how to use one for automotive parts and that's about it.
the good quality one has 2 adjusting dials.
there are 6 settings
30 MΩ 10A
3000
300
30
3
300mV

Then the second dial
DC V
AC V
check
KΩ
DC mA
AC mA

I checked with settings of 3 and AC mA
The other setting AC V doesn't seem right (open circuit)

Using a sears analog tester set at V AC 50 it moves slightly about .2 volts
and about .1 volts in the small tanks.
In a glass of water zero.
Looks like there is measurable current from all electrical items and living creatures.
Some heaters put out too much and lights do too.
Thank you
Terri

I am not familiar with the settings on the first dial but the second dial is more self explanatory. Do you have the instructions with the multimeter. Give me the name and model number and I'll see if I can get the instructions on the net. I think you may have made your connections in error.
I just purchased a multimeter as the old one does not test for ac dc current in milliamps.
This is the way my instructions say to test for current even though they are poor.
It states that when checking unknown current, the red lead always goes into the 10 amp jack and the black lead into the common jack. then the opposite end of the black lead and red leads goes in series with the load. Then you work your way down to the lower amperage until you get a reading. This is done to prevent damage to the multimeter and safety.

Now when I get to milliamps, this is how I make my connections to test the water as per the authors instructions:
Turn off all electrical equipement. Set your multimeter to ac milliamps. Then attach the black lead to the common jack and the other end to a ground, such as the third prong on an extension cord or wall receptical. Attach the red lead to the milliamp jack and the other end to the load (the water). Turn on all electrical equipment and see if you get a reading. If its less then 1 milliamp, then you are ok according to the author but I think that is questionable. For instance; if the meter indicates 0.10 then thats less then 1.0 milliamp provided you are in the proper range.
Now to be sure you are testing correctly, I need to find out what that first dial is about. I cannot say if using that 3 connection is correct unless I see the instructions for your meter.