I personally like AquaClears, and they come with good media that you can reuse for years, not the stupid cartridges that you have to keep buying new ones.
As far as adding it to your current filter or replace it completely, that's up to you and what you want to spend. Biowheels are generally not great with brackish water, the wheel will spit salt spray all over the place. But you can just pull the wheel itself off I think, and use the filter without it.
If you use the two separate filters, they will take up more room on the back and obviously its another thing to plug in and use electricity. But if one should fail, at least your tank isn't without filtration. And you can clean out one filter, knowing that the bacteria is solid in the other.
As far as what size to get.. I like to aim for a total GPH that's 10x your gallons. So for a 20 gallon tank, 200gph total (or more, but that's usually plenty). If you are going to go with a long tank, I'd probably put on two 100gph filters to spread out the current. So, yeah, that's what I'd do
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and my tank now is about 30 gallons but hes only about 2inches long at the mo, but i know hes needs that extra 10gallons so will need a bigger tank within the yr!
