
HI and welcome to the forum.
A couple of questions
How long have you had the tank.
Deepseafisher already asked about the types, and if you post back for him, he is extremely knowledgeable about them.
Did you cycle the tank?
Do you use chemicals to adjust your water conditions?
Now to the mention of overstocked. That is in regards to the bioload a tank and filter system can handle. It is based on the types of fish, and there adult size's.
A 29 gallon tank can house a large number of neon tetras, or guppies, (50 maybe) yet it cannot handle one Oscar.
Bio load is a term for the waste matter in a fish tank. It is made up of a combination of things such as fish waste, (breaks down into ammonia) left over fish food (gain breaks down into ammonia) dead or decaying plant matter, decaying fish scales, and just about any other biological decaying matter in a fish tank.
Ammonia is poisonous to fish, extremely fatal, for some species a level of .2ppm or less, For others they maybe handle up to .9ppm.
Anyhow that is the reference to your tank being overstocked. It is not just the number of fish, but more the amount and type of fish.
Once you post back with the answers to the questions asked, we will steer you down the road to healthy and happy fish, and teach you how not to spend a whole lot of extra money in LFS/LPS along the way. :thumbsup: