Goldfish seem to require a lot of attention when it comes to water quality. A big part of this is filtration, primarily mechanical and biological. The remaining type of filtration, chemical, is often omitted by goldfish keepers who change water so frequently that the chemical purifiers are deemed largely unnecessary. I've been using power filters, a canister filter, and a home made filter. I'll offer my opinions on what I've observed. Note that these are opinions: your mileage may vary.
These are the box filters that hang on the outside of the tank. Most modern power filters pump water into the box, which then overflows out of the box and back into the tank. I currently have two Whispers and a Penguin Biowheel 160. They all seem to be adequate filters; not too expensive, quiet, durable, and easy to clean. The Whisper filters don't have a great reputation, but ours have been fine and I like the fact that the filtration material can be purchased unassembled. That makes it easy to leave out the charcoal.
I have read about many people being very happy with biowheel filter systems. I really can't tell any difference for the 160. Seems like a good filter, though the filter media comes completely preassembled ... you need to do surgery to remove the charcoal, if you plan to rinse and reuse the filter element over and over (if you're careful it will last 4 or 5 months). The tank I have this filter on took 8 weeks to cycle at 65 deg. F.
I have been very impressed by the Hagen Aquaclear power filters. I currently have a mini on my snail tank and two 500s on a 94 gallon tank. I put two sponges in the filters, and no other materials. This will give you both great mechanical filtration and a very good biofilter media. Just squeeze the sponges out in dechlorinated water to clean. The sponges should last for years. If you need to start up a new filter, you can just borrow a sponge from an established filter.
I had a Fluval 303 on a 45 gallon tank with four 3 inch plus ranchu that I was feeding pretty heavily. I will start by saying I think this is a nice filter; quiet, unobtrusive, and effective. But I ended up removing it from the tank. It was just too much trouble to clean. Basically, it needed to have the filter media cleaned every two to three weeks and then needed to be completely broken down every two to three months, cleaning all the hoses and everything. What this means is that if I figured my total filter care time and then averaged it out, the filter needed a net of 30 to 45 minutes of effort on a weekly basis. It was also a great opportunity to inhale tank water every time I lost the siphon.
Sold yet? A lot of people seem to like Ehiem canister filters. I have no experience with them. I personally don't feel a canister is the best option for goldfish.
If, like me, you already have a Fluval, here are some of the modifications I made to make it more suitable for goldfish. I put two foam power head sleeves around the spray bar. This still gave a lot of aeration to the returning water but it minimized the currents, even when the Fluval was on full flow. I also added a plastic pot scrubber over the intake sieve. I cleaned off the pot scrubber every day or so. This provided a mechanical prefilter to reduce some of the cleaning requirements of the Fluval. The last item was to vacuum the tank twice a week.
On the 45 gallon tank, I now use the drilled bulkhead overflow to supply water to a home made filter. This filter is somewhere half way between a wet-dry filter and a power filter. The basic scheme is pretty simple. I bought a five gallon bucket (in HDPE plastic from Hechingers: $5), a submersible magnet drive pump (Maxi-Jet 500, about $15), a basket to hold filter material (I actually bought a closeout stainless steel pasta basket from Williams Sonoma for $15; a $1 plastic water lily planting basket from That Fish Place would work just as well), about 3 feet of vinyl tubing (FDA approved), and a 50 pound bag of lava rock ($10). I also needed about $10 worth of miscellaneous PVC fittings for the plumbing off the bulkhead. I would describe this as a $50 filter
The setup is like this: the bucket is under a tube extending from the bulkhead fitting on the tank. The pump is in the bottom of the bucket with a return flow tube attached to it, that is led back to a spray bar in the tank (I'm using the Fluval's). On top of the pump is about three gallons of carefully cleaned lava rock (for reference, that leaves me with about 45 pounds unused). I then set the basket with filter material (sponge and/or filter wool) in it on top of the lava rock and under the bulkhead flow tube.
To operate, I fill the tank until water just barely starts to overflow into the bucket. Then I fill the bucket to near the top and plug in the pump. This maintains the lava rock mostly submerged, which is why I don't call this a wet-dry filter. A variation on this design is to put the bucket on top of the tank, with a hole in the bottom. Then put the pump in the tank and pump water up into the bucket, where it flows back into the tank by gravity.
I've been very pleased with this filter. It's quick to clean and seems to do a very good job. I recently treated my tank for a full month with Nitrofura-G. I thought I had completely killed the biofilter, but within two days of removing the medication the biofilter was functioning effectively again.
Update: I have now redone this filter with ceramic and plastic biofilter materials, new food-grade PVC tubing, and a plastic mechanical filter material basket. This was to address possible sources of chronic health problems of the fish in this tank. A more detailed discussion is offered under the health section of this web site.
I have engineering training in fluid dynamics and fairly good mechanical instincts. But that didn't save me from nearly emptying my tank out several times accidentally while working the bugs out of my home-brew filter. It is important to think about this for pool or aquarium filter design; try to come up with a fail-safe design.
My first problem was with using a pipe extension inside the tank on the outflow to the bulkhead fitting. The usual sort of thing, a pipe down to near the bottom of the tank with a sieve on the end. But this, in conjunction with the tube on the outside of the bulkhead fitting, would gradually evacuate out all the air in the bulkhead-pipe arrangement over a 20 minute period. As soon as this happened, the water flow out of the tank would suddenly triple, out running the pump and filling the bucket to overflowing.
I solved this by getting rid of the pipe and just taking the water in at the surface. That is, straight overflow. This has the added benefit of eliminating any surface scum that might accumulate.
The second problem occurred when I put the end of the pump return tube into the tank water. In this case, if I turned the pump off, water would immediately start to siphon back through the idle pump. I solved this by just making sure that the tube end was always out of the water.