[lochness]

withholding food is a bad idea - just feed enough that will be consumed in 3 min 3 times per day (their tummies are small) of a high quality brand - like omega, OSI, nutrafin - a serving of frozen brine shrimp packed with spirulina and a serving of frozen blood worms (3 times a week) - get the cubed kind so you don't have to guess how much to feed. Oh, and buy small containers of flake foods and pellets/wafers. Fish foods have a lifespan of about a month, after which they lose alot of their nutritional value. Food deficiency is one of the factors of fish getting weak or sick.

Have you checked your water source? Use the water testing kit on a sample of your water source before you condition it and see what the nitrites/nitrates readings are. My first guess is that your tank hasn't fully cycled, but I'd like some of the more experienced aquarists to reply here (esp the Mods). IMO 3 mos still equates to an immature tank tho this is arguable by some.

Fish require about 10-12 hours of light and your 1.5 inch of gravel is ok - I'm partial to 2-3 inches of gravel since it is a great source of nitrifying bacteria - and vacuuming the gravel doesn't interrupt this. As I said before, I suggest you switch from the Stresscoat to Aquarium Pharmaceuticals Tap Water Conditioner (same company).

Water parameters affect all fish - but the weaker fish or the fish who have a hard time adapting to sudden parameter changes suffer first.

I congratulate you on your diligent water changes and gravel vacuuming - once the tank is fully cycled, this routine will greatly increase the success of your hobby.

Incidentally, what's the temp of the aquarium?

If feasible, I also recommend you buy the Penguin 100 filter and enough ceramic rings to fill it with. Remove the filter cartridge that comes with the filter and simply pack it with the ceramic rings and run it that way - this will supply the tank with some added oxygen due to the additional water movement and a holding place for beneficial bacteria. The only thing you'd clean on this filter is the intake if it got too gunked up - other than that, leave it alone. With this setup, an added air pump and bubble wand isn't really needed.