sand hasnt ruined any of my filters, except my ac 110 wich was not designed for it, gravel will hold besides good bacteria alot of bad bacteria no matter how many filters you have, to clean your gravel thoroughly you will have to move all of your decor around quite often, sand is a very good substrate because any waste that hits your sand will rest on top of it and eventually rise into the water column if you have good circulation. with gravel it will get into the nooks and crannies and stay there. trust me buddy go with sand... and your 125 will not hold 10 caribes if you manage to keep all alive, so im assumeing you plan on upgradeing very soon??