Water Changes For My Goldfish Aquarium

by Steve
(Ft. Rucker, Alabama)

I have two 4.5 inch fancy fantail goldfish in a new 75 gallon bare bottom aquarium. I want to keep the tank as clean as possible and my goal is for these fish to live in a healthy environment.

I am using two Fluval Aqua Clear 110 Power Filters for filtration. I try not to over feed the fish.

How often do you recommend water changes...and what would be a good amount of water to change each time?

Also, if I do frequent water changes, could I safely introduce a medium size black moor goldfish into the tank? Would three fish be too many for the size of the tank?

I don’t want to over crowd or ruin the water quality in the tank.

Thank you.

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Grant's Reply


Hi Steve

Fluval Aqua Clear 110 Power Filters can move a lot of water, I know, I had one. They are recommended for aquariums up to 110 US gallons, so two will be plenty for your 75 gallons.

With just two 4.5 inch (body length?) fantails in an aquarium of this size, they have plenty of room to grow.

Your question regarding water changes is a good one as although you have a huge filtering capacity, water quality does degrade over time, just more slowly in an aquarium of your size.

A lot of enthusiasts make regular partial water changes, and hope they are changing the correct amount.

If the fish don't die, they assume their water change regime is correct.

If they are lucky, this hit and miss method will work for a while, but eventually something will change such as adding new fish, the existing fish growing, or even changing the food, which will cause an outbreak of disease as water quality drops.

The trick here is not to guess, but actually test when and how much water needs to be changed.

All you need is a fresh water test kit available from the better pet stores or specialist fish shops, and start testing. Don't get test strips, they are notoriously inaccurate, get one with test tubes and chemicals that test for ammonia, nitrites, nitrates and pH.

The water parameters you are aiming for are zero ammonia and nitrites, (I assume your filters have gone through the nitrogen cycle), nitrates below 30 ppm (parts per million) and pH around 7.0-7.4.

When your aquarium water starts to move outside these parameters, a water change is required.

You will need to test your water source so you know what your starting point is. For instance your fresh water may actually have a pH of 6.8.

Your next decision will be whether you want to make more frequent partial changes, or less often complete water changes based on the time the water quality starts to drop.

As it happens, the February 2020 Goldfish Gazette discusses this in some detail.

Regarding the question of adding a Black Moor, the only issue is competition for food.

Telescopic eyed Goldfish such as Moors are myopic. In an aquarium with normal eyed fish, these will always get to the food first, or get the majority of the food, and the Moor slowly starves.

If you ensure the Moor gets his fair share, there is no problem regarding room, you have plenty.

Stay safe.