Yeah, I tried using HOB's on my 75 and it wasn't good. Hoped I would be able to get away with it on a 20 gallon long, but it doesn't look like it is going to be the case because just with the HOB off for an hour, the ph got to the right level and the controller shut it off. I'm already trying to figure out where I'm going to cut the hole in the counter top and which cabinet underneath I am going to house the Fluval 405 in. I'm also thinking about getting another 405 for the 10 gallon and running it at 1/2 capacity.