Dying fish!

Question
I've been collecting and enjoying fish for about three years now. I've had my 29-gallon tank for about two years, and over this period I've gone through so many different techniques, trial and error situations, and had to this point considered myself quite educated in the area. Recently, however, I've been having some major problems. At some point or another I've owned just about every different type of fish available at local stores. Right now all I have left are four feeder goldfish. I don't know why, but in the last month I've lost about thirty fish, from rose tetras to platies. I've tried different community situations and it rarely seems as though more aggressive fish are the reason. Rather, it looks as though the little guys are just suffering and slowly dying, sometimes over a few days. I haven't done anything different regarding tank maintenance, though I'm sure I could be more thorough with everything. Money and time are kind of issues when it comes to getting all the perfect chemicals and constant changes, but I'm at a loss. The water is clear, the filter is clean, I bubble oxygen in, the light and temperature are good. I feel like when I go to pet stores they'd rather prove me wrong and try to make me look irresponsible/idiotic rather than giving me good advice. What are some common problems and what do you think could maybe be going on?
Thanks

Answer
Hi Daniel
Saw your question in our question pool.

Just because water looks crystal clear, doesn't mean it is. You won't see the ammonia and nitrites building up in a tank. 4 feeder goldfish is way overstocked for a 29 gallon tank.  

First thing I would suggest doing, is picking up a good test kit-preferably the liquid dropper kind-the dipstick ones suck.  Make sure your ammonia and nitrite levels are at 0 ppm, nitrates ideally should be under 20 ppm.  I know $ is an issue, I completely understand that, but that's one necessity you need to have when keeping a tank.  Take care of the water quality, and the fish will take care of themselves basically.  API makes a good test kit with all the tests you need for about 30. at the store, online at drs. foster and smith it's about 15.00 last time I checked.  Good investment.

Without knowing what your readings are, it's hard to say what's going on.  But that would be the place to start.  Not sure what your set up was before either, if it was overstocked, what fish you had when or how many at each time, so I can't say much at this point.  But, my money is on the tank probably never actually cycled or finsihed the cycle process.  Again, that's just my guess without knowing all the necessary info at this point.  

Here's a great link to read over on the nitrogen cycle or the cycle process as it's referred to, in case you're not familiar with it:

http://freshaquarium.about.com/cs/biologicalcycle/a/nitrogencycle.htm

Let me know if you have more questions.

Christy