I have an idea if a I take a under water pump that has 400 gph and I put some material and activated carbon into a PVC pipe then let the water shoot into the pipe could that work as a filter because it would only cost me $23.86
Definitely not as your only filter. It has potential to work but it could leak, maintenance may be a pain, and if you want ideas for filters search around. Try YouTube. There's DIY hobs, wet/drys ( most popular) and over head filters.