If I were you I would keep the Marineland 400 and just replace the tetra with one of the AC 70's, but definitely keep the Marineland and just stuff it with the cut to fit filter pads that you can by and cut to whatever size you want. You can never go wrong with overfiltering! Just buying one big filter will cut down on maintenance, but what if that filter goes down? Having the additional filtration will help out in such situations and if you ever need to start an additional tank you can just move a filter over to it and don't have to worry about cycling a new tank!