Aquaclear filters are good candidates for HOB DIY refugiums. I've read a number of successful builds with them over time. I also had a very nice mini refugium going with one for the better part of a year with nice fauna. I used a clip-on IKEA LED as the light source. I didn't even really mod the insides of the filter; I just took out the insert and dumped in a bunch of chaeto. If you want a bigger one with any sort of animals larger than pods as a CUC though you'd need to mod it with a grate between the impeller chamber and the rest of the filter (similar things would be needed on most HOBs I've seen). Unfortunately, a power outage caused the filter to stop, and the water level in the tank was apparently too low to get it going again when the power kicked back on. The result was that the chamber to heated up due to the motor and evaporated abnormally fast, so it turned sour in the couple of days it sat unnoticed. That's not just something that can happen with aquaclears; it's a risk with any HOB filter, so you just have to make sure to check that the flow is going and not be absent minded about it like I was. The bigger the filter chamber the less likely that will be to happen too; mine was one of the smallest models and, of course, tiny volumes are less stable when in isolation.
