To plug the hole, first measure the diameter of the hole and then go to a hardware store, garden center, or pool supply shop. Look for a bulkhead fitting. It will resemble the one depicted below.
A bulkhead fitting is simply a few pieces of PVC
(in this case) that screw together to form a plug. A rubber gasket provides a seal against leaks. There is an actual plug that can be unscrewed if necessary. It might come in black which would make it less noticeable (although you could probably paint it). Down the road if you were to have a sump/external reservoir you could use the bulkhead fitting for connecting the appropriate plumbing.
+1 on the filter comment. You could probably add a second filter. I'd recommend the Hagen/Fluval AquaClear series.