You have a few contributers to a nitrate problem in your tank. 1 - lions are messy eaters, 2 - CC
substrate lets big chunks of detritus sit in it and rot, 3 - canister filters have to be cleaned real often or else detritus sits in the mechanical filtration and rots, UG filters are nitrate farms. Reverse flow UG's are a little better, but you're better off without the UG in any form.
How high is high, and what are your other parameters like? A little extra nitrate won't bother your fish too much, but both the lion and the tang are going to outgrow your tank.