Help with system design - 25g

[syncro]

An external herbie would support higher flow rate and be quieter at those rates. Asthetically, it is very clean inside the tank but I don't find it as appealing when looking at the tank as a whole. I really like the glassbox look and even considered lily pipes. Will the overflow above cause any problems at low flow rates?

I'll increase the size of the ATO to 4 or 5g. What headdaches are there with gravity fed ATOs?

Thanks beefOrchicken and FishyFishy.