I had started using these along with my current filters on 4 tanks I am using 2 of the merlin fbf and 2 of the coralife sea storm 100's fbf. I have 1 on 4 different aquariums they are 55 gal tanks. I do like the merlins adjustable features but the sea storm is great also. Does anyone else use the fbf filters? I plain on buying 4 more for my other tanks. The ones I have now have been running for about 1 month now all are mounted to the back of the tank and all my tanks are freshwater. and all tanks are fancy goldfish I have never used this type of filter and I was wondering if anyone had any pro's or con's about them. I plan on keep my current filter set ups on each tank and just adding these as a supplement. AND do you know if there is a certain type of sand I can get at like home depot etc. in case I have to replace it or must I buy the manufactures 10.99 for a 2 1/2 lb replacement sand? Thanks