Choosing filters for fish tanks

by Owen James

Filters for fish like these giant gouramis need to be up to the task!

(Image: crabchick)

There is no shortage of filters for fish tanks on the market. The choice can overwhelm even experienced aquarists, let alone newcomers, with size, cost, water turnover, and filter media all needing to be considered.

All the major manufacturers now rate their filters for fish tank size (in gallons and liters) and also by turnover. But statistics like these only tell part of the story: aquariums are living things, and filters need to be chosen accordingly.

Below are some questions to ask when choosing a filter for fish tanks containing coldwater or tropical freshwater fish. (Marine is different again).

Note: It’s good practice to assume that the manufacturer generously rated the filter under optimal conditions with fresh, unclogged media and with any pipework from the filter being minimized.

Tip: If you’ve a 30-gallon tank, buy a filter rated for 35-50 gallon aquariums, not one made for 20-30 gallon tanks.

1. Do you need a filter for your fish tank?

It’s possible to maintain tropical freshwater tanks with plants and water changes alone. If you go down this route, keep only a few fish until you gain more experience, and don’t even try it with delicate fish like discus.

2. Do you plan to keep big, messy eaters?

Stocking rules of thumb allow you to keep 2″ of tropical freshwater fish length per gallon of tank water, say, or 1″ of fish per 12″ square of water surface. But this makes no allowance for the size and diet of the fish.

Two 12-inch messy oscars will eat a lot more food and produce a lot more waste than 24 one-inch long neon tetras. Bigger fish need bigger filters, with a faster turnover rate, so if you’re going to keep them look for models rated for your tank size and above, not for filters that are only just adequate.

3. How big is your aquarium?

Again, bigger tanks (above four-foot long) need bigger filters to really get the water moving, even if the filter is theoretically rated for your tank’s capacity.

Personally, I’d be tempted to use two smaller external filters in a tank five or six-foot long, rather than one monster sized external filter, or else team up one external filter with separate undergravel filtration.

Small tanks will get away with little internal filters, since most waste matter and all the tank water will very quickly find its way to the filter.

4. What is your budget?

If you’re on a limited budget, it may be better to go for an old-fashioned but dependable filter such as a sponge filter and air pump for a small tank up to two-foot or so, or an undergravel filter driven by one or two powerheads for a larger aquarium.

You’ll get more filter for your money; you’ll also have to be extra vigilant when it comes to tank maintenance.

If your budget isn’t constrained, your choice is huge. Consider getting an external filter with multiple compartments so you can use different media, and look at so-called thermofilters, which include heaters within the unit, to keep ugly equipment out of the tank.

You might even want to go for an integrated system where all the filtration is hidden away, if you’ve not yet bought your aquarium.

5. Do you have room outside of the aquarium?

You wouldn’t be the first person to bring an external filter home, only to realize the only place for it is in-front of the fish tank, in full view of everyone…

External filters require room beneath or to the side of the tank, preferably in a cabinet. You’ll also need room for pipework, and access to a power supply.

Hang-on the back filters need a certain amount of clearance between your tank’s edge and the wall, and can sometimes be hard to fit to tanks with tightly integrated hoods. Check at your store, or on the manufacturer’s website.

6. How often will you be able to clean the filter?

Any filter – internal, external or undergravel – needs to be kept clean. Internal filters will need to be cleaned every week or two to keep them operating at peak efficiency. Undergravel filters will benefit from a vacuuming on a weekly basis if you can find the time, otherwise they can quickly become very dirty.

In contrast, external filters can go 3-4 weeks without cleaning, especially if you’ve got a multi-stage one where you can discard the first stage of filter floss. (Biological media should only be rinsed in spare tank water, remember).

If you’re likely to be very short of time, some of the hang-on filters such as the new Whisper EX provide very quick cleaning mechanisms. Keep your tank slightly less than well-stocked if your time is short, and remember you’ll always need to find time for regular water changes and other tank chores.

7. Do you want to use special media like carbon or peat?

It is possible to use peat plates buried in the gravel with an undergravel filter, or to run carbon inside some internal filters. But if you’re going to be making use of anything other than straightforward mechanical and biological filtration like filter floss, foam, or biological media like bio balls, you’re really better off getting an external filter from day one.

8. Will you be keeping aquarium plants?

Well-run undergravel filters are rarely conducive to good plant growth, since the fast currents through the substrate harm their growth. Also, it’s impossible to use special aquatic soil with undergravel filters – you must use aquarium gravel.

One option is to grow plants in pots half-buried in the gravel. More than a couple of these though, and you’re compromising the effectiveness of your undergravel filter. In general, an internal or external filter is a better bet for plants.

Another point to consider with plants is the filter turnover, and particularly water movement at the surface. Plants require C02 to flourish, and too vigorous filtration will drive CO2 out of the aquarium before the plants get a chance to exploit it, even if you’re supplementing it via a C02 system.

Air-powered filters, venturi nozzles on powerheads and spray bars are certainly not what you want in a planted aquarium. Bigger filters with a slower turnover and all water returns placed well under the surface are a better compromise between creating the right water conditions for fish and ongoing healthy plants.

9. Can you afford a filter from a better manufacturer?

Fact: Not all manufacturers filters are created equal. I’ve found Eheim filters to be bomb-proof, while at the cheaper end the Fluval filters are pretty good, although the build quality is seldom up to the level of Eheim’s. Other manufacters vary from good to disappointing.

You also tend to get fewer bells-and-whistles with some of the cheaper filters (such as the self-priming mechanisms on external filters that stop you having to suck up tank water to get your filter working), so be careful if buying a generic Chinese filter off eBay.

When comparing the cost of filters, make sure you’re also comparing the cost of the filter media. Some filters are sold without including the foam or other media required, which can easily add 10-20% to the price.

10. Will you be upgrading your tank in the near future?

If you’re going to be getting a bigger aquarium one day, it can make sense to get a larger filter now and to turn the flow rate down if required (and allowed), rather than to under filter your future aquarium.

Filters for fish tanks tend to deliver more bangs for the buck as they get higher-rated, so a more powerful filter might deliver 25% more turnover or boast 20% more filter volume, for only a price only 10% higher.

Do you have any tips for choosing filters for fish tanks? Please share them in the comments below!

Check out Katy's Tropical Fish Guide for more aquarium info.

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