Mys, Canister filters are very good filters, but are expensive, fairly difficult to clean, and often times leak if not maintained properly. B's tank maintenence advice IMO is the very best advice
Given on AA and is the same philosophy I use. If you like "hob" filters, I use Aqua Clear 110's, which are more or less glorified sponge filters, as the majority of the filter basket is filled with an oversized sponge.
Hi Mys, I agree with all of the advice given thus far. I have 2 fancy goldfish in a 54 gallon corner tank. I would also recommend the Aqua Clear 110 HOB for you, with perhaps an air powered sponge filter for backup. You could run the AC 110 on the opposite side of the tank (away from your current HOB).
I wanted to use the ACl 110 on my tank, because I had a good experience with an HOB on my smaller tank. Unfortunately, due to the design of my tank, I can't fit a large HOB unless I leave my tank completely uncovered.
I currently run an Eheim 4+350 canister on my tank, as well as 2 air powered sponge filters. Please note: Even with a canister, I still change 40% of my tank water 2x/week to keep my nitrate levels below 20ppm.
Canister filters come with a learning curve. I'm learning more about them as I go.
For example: I recently had a power outage while I was sleeping. (The power strip was inadvertently unplugged.) I later learned that I might have risked poisoning my fish with anaerobic bacteria and/or ammonia when I restarted the canister. Amazingly, I did not lose my cycle, and my fish eventually recovered from the stress. Since then, I've installed battery back up air pumps on one of my sponge filters. And after much research, I've concluded that during an extended outage (>1-2 hours) I will need to open up the canister, add the biological media to my tank, and run an air line beneath the media...I would much prefer to deal with an HOB filter during a power outage.
(However, if you have a power generator at home, you may be able to avoid this problem altogether.)
The good news is that the Eheim canister seems really solid. I was lucky enough to find it on sale at PetSmart, so I think I made a good investment. But it is still cost 2x what you would pay for a large HOB. There are less expensive canisters, but if your dad can build a canister for you, that could be the best deal yet. (And it would probably be sturdier than an HOB.)
Mys, you asked about the water flow. It sounds like you have a well planned environment for your fish. My goldies have not had any issues with the flow being too strong. However, my canister came with a spray bar.
If your dad builds a canister for you, ask him to include a spray bar, which will spread out the clean water output so the flow is not so powerful. (Or you can look for a spray bar to add to most canister filters.)
Good luck, and have fun with your tank!