Another option is in-line pond filters, these come in all kinds of sizes and can have UV filters etc too, that would be the cheapest ways to have a big filter system. Practical too, particularly the in-line pond filters, you can add these too and have 2-3 etc to go really overkill. 2 pond pumps with bit LPH, running from one end of the tank, in-line filters above the tank and then coming back in at the opposite end would give a nice current too if you wanted to go for a river effect with reeds, proper swimming fish etc. You can replace the filter media in these in-line boxes with whatever you like.