I've had fish pecking at the old fertiliser tablets I used to use years ago and they never seemed to have any problems from them. If you're worried then just shove them under the substrate so the fish can't get at them!
well if that was the case, i would go really heavy on the substrate like i am going to do, i am going to use sphagnum moss peat, and possibly normal peat or vice versa (see my post please! ) and then use JBL aquabasis mixed with osmocote and then maybe if i feel up to it, perhaps mixed with florapol from JBL. then capped with akadama (double red line of course!) then places in between the islands and at the foreground will be black gravel to help contrast with the glosso.
but changhing the substrate will be hugely messy with a planted substrate, i currently have large pea gravel and you could imagine that draining out 600litres of water into buckets would take some time to do and then again, i have nothing to do with the fish
they are rather too large for 18'' tanks or buckets lol.
it would be a case of about 15 bags of eco complete (very expensive) or smaller gravel and root tabs/balls i think.
unless there is a cheaper planted tank alternative that wont mess up the tank and jam the filters up?