What Size External Filter Do I Need?

by Jackie
(Australia)

I have a 130 litre tank and two goldfish, one is almost the size of my palm and the other a bit smaller. I brought a new tank and it has been running for about 2 months now the filter that came with it can't handle the goldfish waste and the water is always cloudy, so I do water changes almost every second day.

I would like to get an external filter and have been reading that getting a filter that runs water out more would be better. So instead of a filter for a 130 litre tank get one that can filter a 520 litre tank for better filtration? Is this true.

Because they are leaving a lot of waste which this current filter doesn't do much to help with getting rid of it. And should I get one with UV as I think I have a bit of algae that grows along the edges. Thank you.

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Grant's Reply


Hi Jackie

Let's start with the basics.

Quoting from my Aquarium Filters page The purpose of a filter is not to make the water clean and clear. That is only a secondary benefit.

A filter’s primary purpose is to process the waste generated in the aquarium.

Is your filter cycled?
This means the aquarium has gone through the nitrogen cycle, and the filter is processing waste properly. You know this by testing for ammonia. If there is ZERO ammonia, the filter is doing its job.

To confirm the filter is the correct size, the water flow rate should be rated at double the aquarium volume per hour. Unless your present filter is an internal sponge filter, most filters have a L/hr rating (litres pre hour). Yours should be rated at at least 260L/hr. This rating is the best it will do when first installed. I usually go for double that so a 520L/hr filter (approximately) would be better.

As for the cloudiness, that may be free floating alga which can't be filtered out by most filters. Is the aquarium getting too much light?

A filter with a UV clarifier would get rid of the free floating alga, but it won't stop it from growing on the sides of the aquarium. It will help reduce it, but not entirely eliminate it.