I got this ~30 gallon hex tank for free but it needs a filter. It's going to be primarily goldfish and kid (cheap and flushable) stuff so the filter doesn't need to be elaborate, just needs to fit. Please guide me to something cheap and that will fit. Everything I've seen on ebay hgs off the back of the tank? I guess it hangs over the edge by the same bracket that pumps the water back in? This is new to me. If more dimensions are needed, fire away