I really like the look of sand in my tank and want to keep this when I upgrade my tank (120l arrives on Sunday). Now I've already got the sand (being a student I picked up the cheap kids play sand from Argos), last time I put pond soil underneath it. This works ok but because it's soil it eventually works its way to the surface, especially during rescapes! What I want to know is what's the best way to proceed, do I find an alternative substrate to go under the sand, one which is less biodegradable and less likely to mix or should I go with just sand and get some root tabs for my crypts etc. Unfortunately I do have a rather tight budget for this (I probably wouldn't even be doing it but the seals in my old tank have become suspect and there was a barginous one on ebay) so I really don't want to end up spending a fortune. Any advice greatly received. *edit* Forgot to add that since I'll be replacing my old tank this will be an in-and-out job so substrates that leech ammonia are probably out of the question.