I have been following this thread and the following are several item based on experience or research of the literature which I believe to be pertinent.
The fish are pretty disturbed when i do a vac.
Mine were also when I started my approximately monthly 25% WC's due to a "good gravel vaccum" but they have "gotten used to it".
I would but i only have the amonia and the nirite.
If you are fairly new to the hobby, as I am, I can recommend the the TetraTest Laborett and the TetraTest NO3 test kit.
These kits are inexpensive and have "been a ton of help" to me while gaining experience.
I do a 50%+ water change each week. Sometimes my fish don't mind, other times they pout for a day or so.
Per my experience (please bear in mind only 8 months worth) 10% daily WC's are significantly preferable to a 50% weekly WC (although they are not equal).
The fish are "pouting" because they do not feel good due to the instantaneous change in water parameters.
Another key is temperature. My fish almost revolt against me when the temperature is higher than the water in the tank. I always shoot for about 2 dergees below tank temperature for my water changes.
I have found this to be very true also when I do a 25% WC and refill with tap water.
One other thing is you are changing a little too much if you are doing 75% of the water. Unless you balance your KH and GH, the swings in both can be very stressful.
IMHO 75% is way, way too much and as set forth in the remainder of F4A's post the instantaneous change in water parameters is "really stressing out" your fish.
to tell you the truth i dont dose anything. No ferts,co2,decloranizers,nothing.
In general this is good!
Unless you have a high end water softener (ie. RO) you are probably inducing serious salt into your tank!
The chemicals which you are putting in your water softener are only converting (in it's most simplistic explanation) calcium chloride (CaCl) to sodium salt (NaCl) or potassium salt (KCl).
First, my neons do the same thing. When the lights come on they look like ghosts but color up during the day.
My neons and glowlights have the same very, very nice coloration when the lights first come on!
Gravel vacs are extremely important
In my experience gathering I had a serious ammonia and nitrite spike mostly due to "gunk" which had built up but which was not observable from the front or sides of the tank.