I had a wp40 in a 100g and it was crazy strong. I couldn't even let it be on to adjust the settings without it creating a sandstorm instantly. So I got a wp25 and even that I can only use on certain settings so it doesn't blow away my sand. I have carib special grade (larger grain) sand too. Btw rw15=wp40. That is too much for a 55g IMO. I have read the rw series are more adjustable on the low side but without 1st hand experience I can't speak on those for sure.