The undergravel filter is a problem right there. The undergravel filter does work, but not as effectivly as a regular filter. It filters the water but to just blow the debris around the tank. It doesnt elimenate the waist's that it is sposed to. i would go and get a filter that is right for your tank size, for the fish's sake. The way i see it is would you like to swim in your own poop ;)...haha.
Sorry about not being to specific. But what i mean by water stats is a test kid ( preferably MASTER TEST KIT ) which tests for kh, gh, AMMONIA, NITRITE, NITRATE, AND Ph.
Have you cycled the tank? Here is what cycling the tank means in a non confusing way:
Cycling a tank is pretty simple when you brake it down. The point of cycling a tank is so when u put your first fish in they dont die. When the fish poop, it turns into AMMONIA which is VERY toxic and kills the fish. If your going to cycle a tank with fish than get a few zebra danios or platy's/mollie's they are all veyr good starter fish to cycle the tank with. So, the poop it turns into ammonia, and then there needs to be bacterial colonies/bacteria in the tank which eat/consumes the ammonia, which turns into NITRITE, which is toxic to the fish also. The point of cycling the tank is to make bactera colonies to eat the ammonia and nitrite. When that happens, than it turns into NITRATE which is less less less harmful to the fish and you want to maintain it under 40. You know your done cycling the tank when you have 0 AMMONIA, 0 NITRITE, AND UNDER 40 NITRATE. I hope this makes sense I tryed to break it down so it would be easier, because it can be confusing!
I hope this helps you out!