I agree. Compatability and enough filtration are the keys to all aqauriums. Along with water changes,and not overfeeding. In my view ,,to answer the original question ,the tank is overstocked and therein lies the problem. Many people begin stocking their tanks and may or may not have a filter rated for the volume of water in the tank. Some of the flow rates for filters do not take into consideration the flow rate after filter cartridges or other media is installed which may be slightly less than the advertised flow rate. It is always better in my opinion to overfilter the tank for that reason. Filters in essence, slow the deterioration of water quality that begins when fish are introduced to the tank. The more filtration, the more effeciently the dissolved or floating particles are removed and trapped in the filter media ,which is why maintaining your filter is mandatory for the health of your fish along with water changes on a regular basis. The filters only remove what is floating in the water, they do not clean the decaying matter that collects on the bottom of the tank. the more fish you have,, the more waste that is produced. What goes in, must come out. Heavy filtration will help keep the debri in the water column longer so that more can be trapped in the media and this media,, should be cleaned or replaced as it becomes less effective. Too many people over stock, and over feed, and thus you can never really have too much filtration. As for "how many fish can I fit in my aquarium?" If you need to ask,, You prolly need a larger aquarium. Many folks know that the fish they have will get large ,and that they may have too many, and often ,they indicate a larger tank is in the future. But sadly,, for whatever reason,, this seldom happens or at least not to the degree that one would like.
The most important medication in your fish medicine cabinet is.. Clean water.