The filtration is less of an issue than you think: a filter either removes ammonia in real time or it doesn't. If your filter lacks the turnover and/or the bacteria to remove ammonia as the fish produce it, then you don't have enough filtration. Add filtration capacity beyond that and you aren't getting "cleaner" water as such, just more circulation, and likely better mechanical/chemical filtration.
What makes a difference in terms of water changes is stocking. If two fish produce X nitrate in one week such that you need a weekly water changes to reduce the nitrate to safe levels, then if you only have one fish *in the same sized tank* you'll have only half the nitrate per week and therefore you'll only need to do half as many water changes.
In other words, if economy is an issue, look to stocking rather than filtration. Get a big tank but under-stock it.
I'll also make another point. Brackish water fish are less fussed about salinity than they are water quality and water chemistry. Making up a 'home brew' Malawi salt mix is cheap and easy, and that'll buffer the water very nicely against the pH changes brackish water fish hate. If you use hard, alkaline water at SG 1.005 rather than tap water at SG 1.010, you'll get acceptable results without much cost.
A protein skimmer can also make a big difference by removing organic chemicals before they get turned into nitrate.
One last thought: if you know anyone with a marine tank, don't be afraid to re-use their reef tank water in your brackish water aquarium. Mixed with tap water, the results will be more than acceptable for most brackish water species. Reef tanks need to be kept at nitrate less well below 20 mg/l, but that's fine for most fish-only brackish systems.
Cheers, Neale
hi i am looking into setting up a 5 foot barckish the only thing i need to ask is how often should i do a water change would every 2 weeks be ok with the right filteration ? it would prove very expencive doing a water change every week cheers phil
