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One subject I see come up a lot is filtration, especially whether hang-on-back (HOB) or canisters are better. This is a tricky subject since it is partly a matter of opinion and most people will simply recommend whatever they do. However, I have come to the realization that in most situations there is a clearly better option.
If you just look at the filters themselves canisters are better. They have a higher media capacity and higher flow. They also have very customizable media, you can use pretty much anything. All of these effect filtration in a very positive way. However, canisters are a huge pain to clean. It is usually a long project that gets put off for months under the misconception that unless the flow is reduced it is fine. In reality this means all the debris is rotting and ruining the water quality. I have personally seen tanks go from 80ppm nitrate down to 10ppm just by cleaning a neglected canister. They can do a lot of damage just by running if they are not maintained properly. Cleaning them usually means dragging them out to a utility sink or bath tub and spending 30-60 minutes cleaning them. This includes rinsing the mechanical media, replacing carbon, cleaning the trays and inside of the canister, etc. If the tank isn't too heavily stocked and you really stay on top of the maintenance then the cleaning time can be minimized, but it is still significant.
HOBs on the other hand in general have lower flow and much less media. Most have nothing more than a flimsy slide-in cartridge that is overpriced. However, Fluval AquaClears have much more filtration than almost any other HOB (and a lot of canisters for that matter) and have MUCH more media than other HOBs. If you lay out the media from an AquaClear110 and any other HOB out there it is a joke how little media comes with the other filters. It is truly pathetic. So if you are going to use a HOB do not waste your money on anything other than an AquaClear. Best of all is that you can use the media in an AquaClear over and over again for years. They have the same type of biomedia and foam as a canister so you get exceptional filtration without having to pay for media every month (or even every year). You do of course have to replace the carbon if you use it, but it would be better in most situations to replace it with either more biomedia, Poly-Filter, or Purigen.
The real difference between canisters and HOBs is the real world use. As stated if you just look at the filters the canisters are a clear winner. But because even the HOBs with the most work can easily be cleaned in 5-10 minutes top it actually happens. Because canisters are almost always neglected they simply don't have as positive of an impact on the water quality. Because HOBs are so much easier to maintain and it actually gets done they are the better option in most cases.
I had to stop using HOBs on my clients' tanks. I also had to stop using sand. Both were simply too easy to maintain. The sand I use is no maintenance and the HOBs were done in 5-10 minutes. This doesn't look good to people who pay you to do all the cleaning for them. Some get too tempted and want to do ti themselves. Others will comlain about the price for so little work. So instead I use gravel and canisters. When they see how much work the canisters are to clean and all the stuff I vacuum out of the gravel they are happy to keep paying me to do it so that they don't have to. This just shows how significant the difference in maintenance is.
If you are going to use a canister use a Fluval. They are the best on the market. I have used almost all of the canisters on the market and they are definitely the best. The only thing they should have but don't is a carrying handle. They do have things I would miss if I couldn't use them anymore (tank rim adapter, best intake strainer out there, etc.). Eheims are good, but in my experience not as good as Fluvals. They got their reputation back when no other companies were making decent canisters so it was easy for them to stand out from the rest (and they are good filters). Now though there are a lot more decent canisters out there and Eheims are really nothing special anymore. They still have the reputation, and the price to go with it, but they really aren't anything special. There are a lot of other brands out there, some better than others, some so bad it is hard to believe they ever looked at any other canisters when they designed their own, but don't waste your time and money on anything except a Fluval. On tanks under 75 gallons I would do a Fluval 405 or 406. Above 75 I would just do an FX5 or FX6. If you are right at 75 gallons or so and the stocking will be light (just community fish) a 405 or 406 should do the job.