instead of gravel you could also make a false bottem by cutting egg-vcrate to fit in the bottem of the tank and supporting it with short pieces of PVC pipe. then put a screen on and whatever substrate you decide to use.
gravel is good, cheap and easy. but eventually, substrate and small particles WILL sift down into the drainage layer. gravel will clog easier then an open bottem which could be cleaned. if your building a smaller tank and ever have to move it, that gravel will be quite heavy.
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