Thanks everyone for the advice so far...
As I suspected, the Tang is out until I get a bigger tank... The dry rock is the way I'll go, I think. The dry fijii rock from bulk reef supply looks
really nice and would be my preferred option, but the shipping was going to be almost twice the price of the rock! So I guess that I'll end up with the south seas lace rock that I can get from in Canada and shipped cheaply...
I guess I hadn't really thought too much about the clean up crew, which is probably a mistake. I have to admit that the thought of ending up with 20-30 snails and 3-5 hermits in a 25 gallon tank is blowing my mind a bit - I don't know whether there'll be that much to clean up!
With regards to the filter, I guess you're worried about it becoming full of nitrates? There are not any bio-balls or anything like in there, just a pair of what look like heavy duty nylon scourers that provide some mechanical filtration. The guy at the LFS has already suggested replacing these with some that are impregnated with carbon. I guess I can always keep swapping them out one at a time if they start to get too clogged up.. Does this sound reasonable?
Thanks,
Robin.