07-19-2011, 08:20 PM
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Diagnosing a health issue is often very difficult. Internal problems are usually undetectable until the fish just dies. Prevention is the best "cure" and this is mainly achieved by avoiding stress as much as possible. Fish experience stress from many things: water parameters, other fish in the tank that may be less than suitable, overcrowding, excessive water movement, light-coloured substrates, too much bright light, noise, high nitrates, etc.
There are several things that I can see causing stress. Individually each may not be too great of a factor, but collectively it only gets worse. You asked for help and I will do my best to provide it, so please don't take this as just criticism of this and that. The fish's health is the concern.
One thing that stands out here is temperature. Neon tetra are cooler water fish by which I mean low to mid-70's. They will literally burn out above 77-78F. A degree or two may not seem like much, but to the fish who must remain in the water permanently with no means of escaping, it takes its toll on their metabolism and this is stressful. Red Eye Tetra would also be better slightly cooler. Even the Pristella would be better in the mid-70's. I realize the temperature range in the profiles for these latter two tetra goes to 79F and 82F respectively, but there are other factors at work in this. Loaches can tolerate higher temps but don't need to, so 77-78F would be better all around.
The second problem is that tetra are shoaling fish, as indeed are loaches. These fish must be in groups, the more the better, though for tetra 6 is usually considered minimum. Not only is there security--either real or perceived--for the fish in numbers, but these fish often have a social structure and interactions between members of the group that they naturally expect. [I am sure you have witnessed this frequently with your loaches, one of the most social fish in aquaria.] Being on their own or even in 2 or 3 denies them what nature intended, and that is highly stressful.
However, we come to another problem here, tank size. I realize your clown loaches appear to be doing well in a 40g, but that is really too small a space for a fish that can (or should) attain more than a foot in length. It may not be evident, these things usually are not, but long-term this is hurting the loaches. There is so much to this aspect i scarcely know where to begin, so I will leave it. Five is a good group for loaches, but this species should be in a 5 or preferably 6-foot tank. You can read more in our profile, click on the shaded name Clown Loach.
You should be doing much more frequent water changes. You don't have live plants (these would help with the water quality a lot) so there is no way of removing the buildup of toxins and pheromones from all these fish except more significant water changes. I would recommend half the tank volume every week, no less. Some others may suggest more than once a week in so small (to the fish) a space, and I couldn't argue. But it is best to work up in stages.
How exactly is your well water unstable? Another source of stress is adding chemicals to a tank, and I am not a fan of using water adjusting substances in an aquarium with fish. If the water must be prepared first, do it outside the aquarium and then add consistent water at each water change to avoid fluctuations in hardness and pH and help maintain a biological stability. The fish you are maintaining are all soft water, very soft water, and leaving it soft would be better than fiddling with it. Let the pH drop if it likes. With regular partial water changes of 50% weekly this will not be an issue. It isn't in my tanks, and I have a lot of fish in them with water that is < 1 dGH, 0 dKH and pH at 5 in some to 6 in others. All wild caught soft water fish.
I hope this will help.