I've got 3 HOBs at the moment in my 29g. But that's only because I'm "seasoning" a new one. Once that one is ready, I'll decommission the oldest. I started with one that came with the tank, it's old and the flow is too slow without ability to regulate it, so I added the second one thinking I'd replace the original, rhen decided to leave it there just because. This third one is a brand new unit I got for free. I'll remove the oldest one once it's all said and done. The two filters work fine for my tank, I don't think you can have water that's "too clean" by having a second filter. If anything, I've noticed this seems to help me extend the time between water changes for this tank.