Advice sought: Is the water my problem?

Or is it just me? Please excuse a long post, but I am seeking help, so will give all the info I can think of.

I've had my 75 gal freshwater community tank going for over 6 months now, but am still losing fish for no reason I can understand. They just die frequently, sometimes lethargically, sometimes unexpectedly. Ive tried melafix and trichlorphon, but they still die.

The fish: 1 red tail shark 3', 5 sailfin mollies 2', 3 gouramis 2', 5 glass cats 2', 5 barbs 1', 3 platies 1', 3 guppies 1', 2 danios 1', 12 medium neons, 2 clown loaches 2', 2 plecos 3', 2 kuhli loaches 2', 2 corys 2', 3 apple snails 1'.

The filtration: 1 Fluval 4 plus internal filter, 1 550 L per hour filter that is part of the hood

that is rather soft water, at least by my standards. I'd stop trying to keep livebearers until things settle down. The water doesn't have much buffering capacity, so the pH will drift downward as metabolic products from the fish build up. More frequent water changes would be the first step. You don't mention which fish are dying, I don't think the water would be good for the mollies, they can be a problem by themselves. Plecos can be hard to get started, I don't know if it is keeping conditions or their health when purchased. I went through several, the ones I have now seem to do quite well. My experience with gouramies, they are not nearly as peaceful as they look. I tried a mixed tank of gouramies, they ended up killing each other.

Other than something mysterious, like having soap on your hands when working in the tank, that's all I can think of. It sounds like a rather nice tank, the setup you describe is quite good.

To fast to remove DOCs. You need to pre-filter and clean them (pre-filter) every other day, plus gravel vac and change water more often, or, add a bio-wheel.

Feed smaller amounts, more often. I suspect your over feeding.

Because of the build-up of DOCs. You could also use a phospate remover such as Hagen Green 'X'.

Your removing solid waste when you gravel vac, which is equally spread over the intire bottom of the tank. The solid waste in the 50% you didn't vac, decomposes and produces many organic and inorganic compounds. Your tank has become organic laden - the fish are suffering from organic pollution.

Are you sure your water has chlorine and not chloramines. Something like AquaSafe (by Tetra) removes both - also de-toxifies toxic metals, adds electrolytes, etc.

One of the five points of defining water quality is; stability, *not* fluctuation. A ph drop of more than two tenths in 4 weeks means the water's buffering capacity is almost gone and it's time to increase kH. The waters buffering capacity is due to carbonates and bicarbonates, which gives the water ability to keep pH stable. Add enough sodium bicarbonate (baking soda) to raise the kH to 5 and put a marble chip in the filter box.