Only the bottom. Unless otherwise noted, most 55's are totally fine to drill. I drilled my last tank which was a 75, and it was only tempered on the bottom.only thing i would be worried about is tempered glass. i heard that 55's are all tempered glass and cannot be drilled. if you are thinking of doing any drilling on it
I have a 92 gallon frag tank,2 40 breeder, and 275 gallon pvc tub fuge plumbed together. Best thing I have done is add that tub to my system. Parameters are rock solid. Plus I have an oversized skimmer, tons of sand, rock, and I grow mangroves. I also banish naughty fish to that tub and dont feel bad because its a huge tub.