My HOB filter broke today. It was a Top Fin filter for 10 gallon tanks, came with the kit. Is Top Fin the cheap brand? It's only been running about 3 months. Luckily, I was able to buy a new filter before the pet store closed tonight. This time I went with the Tetra HOB filter. It's a lot like the other one, except it has an extra filter sponge. While I was setting it up, it said "not reccommended for aquariums with sand." Well, I have sand- could that be why the other one broke? Will I constantly be replacing filters if I keep getting this type, because of the sand? I had an extra sponge wedged in the old filter, so I moved that one over, squeezed it in with the new filter. Another question, how long should I leave the old sponge there before I take it out? Is two weeks about right? I tested my water tonight- ammonia 0, nitrate 10, Ph 7.6. So it looks like nothing is off from where it was. I had to do a water change because I messed up removing the old filter, and got lots of gunk in my tank. It looks much better now with the new filter going, and some new water.