I probably paid too much for it (i.e. this was probably overkill) but I replaced my old failed mechanical water filter in my fish tank with a new, fancy, Eheim canister filter. Supposedly this thing will support the nitrobacter and nitrosonomas which are what convert the toxic ammonia peed out by the fish into relatively harmless nitrates... in other words it will serve as the biofilter as well as the mechanical filter. I'm somehow skeptical of this and am glad the well-established gravel filter with the two powerheads is still working as the primary biofilter.
The old filter had been broken for some time, so the tank had gotten really nasty, with a lot of algae buildup on the side and some pretty cloudy water. I scrubbed the algae off the side of the tank (this is a really gross job) and after installing the new filter, cranked it up all the way, and within an hour the tank was completely clear. The fish seem much more active, too, so hopefully it's more to their liking.
I have five fish remaining in the tank, after losing three over the years. But the five I have left are 11 years old now, which I think is not doing too bad for a total amateur aquarist. I'm kind of proud of that.
The only scary thing is that this filter sits on the floor, underneath the tank. Siphon action pulls water out of the tank into the filter, and then a pump pushes it back up a return tube into the tank. What's scary is that if any of those tubes ever come loose, or if the canister springs a leak, the siphon will simply drain the entire tank onto the floor. Not good for either my house or my fish! The tubes are pretty large diameter, too, so this wouldn't take but a few minutes. I've been watching it carefully for any propensity for the hoses to detach or the filter housing to leak... so far so good. Presumably they thought of this rather obvious and disastrous failure mode and engineered the product appropriately. Eheims have a good reputation.