I am not going into huge detail here just trying to give some useful information to help others understand why someone says the tank is overpoulated and why water changes are so important.
First, the rule of thumb is 1 inch of of the adult size of the fish per gallon of water. A 10 gallon tank has about 8.5 gallon of water in it after you take away the water loss for the gravel space and the fact that that most are closer to 9.5 gallons instead of a true 10 gallons when full without a substrate. Plecos need a 20 gallon tank for space and the fact that they produce more waste than the normal fish because of their vegetative ways. Neons are smaller and you can break the rule somewhat but they also need room to swim and school together so even 10 neons can be too much for a 10 gallon. Amphibians need at least a 10 gallon tank for themselves with no other animals in the tank. An Oscar needs at least a 50 gallon if not a 75 just because of their growth needs and the recuring fact they produce a ton of waste. There are other fish that have specific needs so ask about them and we will do what we can to help you get the "proper" setup for them.
Also remember that the 1 inch per gallon is the adult size of the fish. So if you have 25 baby swords in a 10 gallon, the tank is overpolulated.
Then there are water changes. I have seen some falacies on the site that some think that water changes are not needed just because there is no fish waste in the gravel because of overfiltering. This is not true. If you do not perform regular water changes, dissolved solids such as coppers, salts and not just table salt but any salt inculding Sulfate salts and Phosphate salts not to mention other dissolved solids. They will build up and cause fish stress and death ever though all the parameters are perfect in the tank. An unexplained fish death in a tank can often be attributed to dissolved solids and we will never know it because we can't reliably test for most of them. In nature, the water is changed by the minute, weekly water changes are the least we can do for our fish to maintain the cleanest water possible and prevent buildups of dissolved solids.
I hope this helps a little and encourages others to post a lot more questions about stocking their tanks and how to properly maintain their tanks for the benefit of the hobby and the health of our fish.