Goldfish Aquarium Water Testing

Testing water quality in a goldfish aquarium or pond is an essential element of routine maintenance. It provides advanced warning of problems, reassurance that problems are less likely, and elimination of possibilities when diagnosing a problem.

There are 4 main aspects of water quality to test, all related to each other chemically.

Ammonia and Nitrite

Ammonia and nitrite levels can build up to dangerous or even fatal levels in a new tank. Ammonia is present in goldfish faeces, and is produced by rotting plant material and uneaten food. Certain types of bacteria feed on ammonia, turning it into nitrite, then nitrates. Until these bacteria are present in sufficient numbers, the ammonia and nitrite levels will rise.

The ideal levels of ammonia and nitrite are zero, but small amounts (less than 4 parts per million) can be tolerable. However, in a new tank with no ammonia-feeding bacteria the ammonia level will peak after a week, with the nitrite levels peaking a fortnight later. It is very important to have chemicals to deal with these toxins at hand during the first month of the aquarium.


As mentioned earlier, nitrites are eventually converted to nitrates, which are much less toxic to goldfish. Nitrates are still dangerous, and although goldfish that have been in the aquarium for a long time can build up tolerance, new goldfish will struggle to survive in water with high nitrate levels.

Regular, adequate water changes should be enough to prevent nitrates building up to dangerous levels. If testing shows high nitrate levels, either increase the frequency or the volume of water changes.

pH Level

pH is a measure of acidity and alkalinity. The scale runs from 0 (most acidic) to 14 (most alkaline) with 7 being neutral.

The optimum pH for goldfish water is slightly alkaline, but goldfish are happy with anything between 6 and 8. The most important thing to remember about pH management is that changes in pH are more harmful to goldfish that being outside the optimum range. Stable pH is the key.

Water Hardness

Water hardness is important because it regulates the pH of the water by a process known in chemistry as buffering. In practice it means substances dissolved in the aquarium water absorb the impact of changes that would otherwise change the pH of the water.

There are two measures of water hardness: general/total hardness and carbonate hardness. It is only necessary to measure one or the other, not both.

General/total hardness is often expressed as dH, GH or dGH. It should be at least 4dH to ensure buffering works, and goldfish can tolerate levels up to 20dH.

Carbonate hardness is expressed as mg/L and should be between 70mg/L and 400mg/L.

Testing Routine

Testing for ammonia and nitrites should – in theory – be unnecessary in an established goldfish aquarium. Testing for nitrates should be done with every water change, i.e. weekly. The same goes for pH and hardness tests.

The other time to perform testing is when something appears to be wrong: goldfish not eating, one dying, new fish dying and upon discovery of illness or disease.

It is also very important to record the results of testing. If there is an outbreak of chilodonella for example – which can be caused by poor water quality – it is no good knowing what the pH of the aquarium water is on the day of the outbreak if you can’t remember what it was in the days and weeks running up to the outbreak.

17 Responses to “Goldfish Aquarium Water Testing”

  1. marion Says:

    I have a 20 gallon aquarium. I had not cleaned my tank for about 3 weeks. As i was cleaning the tank, today, I noticed there were dark greenish spots stuck to the sides of the tank in different places and they were hard to clean off, would you know what it is and is it bad stuff?
    There were blooms of brown alga growing also which wipes off easily. I am in Oahu, Hawaii.

  2. admin Says:

    Hi Marion,

    Can’t say for certain what this is, but it’s probably not serious. Green algae can be hard to wipe off – better using a razor blade to scrape it off.

    I hope everything is ok with your tank now.

  3. Adam Says:

    I have a goldfish who sits on the bottom of the tank, or on the plants, doesnt swim much, when it does it looks like a lot of effort, has developed dark marks in her tail, yet eats just fine and none of the other three fish in the tank are behaving in the same manor

    I have changed the water a but hasnt made mauch difference and using a revitaliser tonic isnt having a noticable effect

    Any ideas?

    many thanks


  4. Daniel Says:

    thank you, your website has been a lot of help to me, keep up the good work

  5. marie Says:

    I have the same problem as Adam above. i had one fish who just seemed to lose interest in everything he stayed at the bottom of the tank in his log and eventually died. I have just introduced two new fish and one of them is just sitting in the plants wiggling slightly but not swimming. he looks very sluggish what could be the problem?

  6. Matthew Says:

    I sounds like you might have a bully fish on your hands. Best to move them in seperate tanks for a few days and see if there behavior has changed. Also a its a good idea to look carefull if any of them suffured an injury that might be infected, usually more by fungus than bacteria (you will see white velvetly or fuzzy like patches or spots on fins or tail or around mouth or in a wound). It is a slim chance but possible that they have a parisite but need more info to fugure ot that one.
    It sounds like that Adam’s fish’s tank either has a very low ph and/or high ammonia levels in that case change the water a couple times and look into a better filter or some ammonia neturlizer and ammonia test kit and a ph raser and ph test kit (a tiny pinch can of baking soda can rase ph some but not recommended to be used more than twice in a water change). But theres a good chance its Gill Disease: Fish struggles to breath, stays at the bottom, loss in appitite folds up all other fins and swims with one on one side; all due to a bacterial infection in the gills cause by excessive ammonia, poor conditions… a very tough one to diagnose at first because there are little to no physical signs. but there are still may treatments in pet stores and also here is some web sites diagnose and find meds to treat such disease u don’t nesicarily need to buy them from them you might be able to find what u need in a local pet store…
    and of course this site your on

  7. katherine Says:

    i have a goldfish that has all the symptoms of fin rot she hovers around the bottom
    and has pink streaks on her tail with rough edges on the tail she also looks quite fat
    and has see through balls hanging from her tummy right up to under the tail is this part of the illness or could she be carrying eggs also how do i test the water.

    thank you

  8. Jennifer Says:

    Having the same thing with the see threw things by the tail they do have fin rot and are be treated for it as wee speak !

  9. Sam Says:

    I have a fish wth black dots on its gills,sides and fins. Could this be Ammonia poisioning or a different disease.

  10. hope Says:

    i have a 2 and a half gallon fish tank with 3 gold fish in it. is it all right for the pH level to be 5?

  11. damian Says:

    hi i got 10 gold fish in owe pond and now we only got to left and i got the pump going on timer and it gets filled up with rain water i checked the pH it was up in 8 so i put ph down now near 6 and i don’t like to see the fish die

    thanks damian

  12. Lily Says:

    I have 3 goldfish in a small tank as they are all waiting to be relocated to a larger tank very soon.

    I think my housemate over-fed the fish when I was away and the water was really cloudy on my return. I changed the water and not long after one of the fish had large black patches on its body, tail and mouth.

    I think this is probably amonia burns recovering, however it has been nearly 3 months now and the black patches have not disappeared. Plus, the fish has now got a ragged tail and one of its fins on its tummy is rapidly disappearing!


  13. Kaylie Says:

    I recently set up a 150 gallon pond in my yard and bought 5 fish. I have a pump but the water is a bit murky. The fish barely swim at all and the pH level is almost alkaline. I’m not sure if it’s worth using treatment because the workers at the pet store say it can just make them even more sick. What should I do?

    thanks a lot

  14. Sally Says:

    Hi my kids got 2 goldfish for xmas. One morning i come down and found them playing with the fish they had removed them from tank and one has lost half a tail. They both feed fine but wondered if the tail returns.

    But today i noticed both have got black patches on them one more than the other, Please could anyone let me know what it might be.

    the patchers are more like a rash than spots,they eat fine and they swim around alot too. If anyone can help at all i would be grateful as quite worried at the mo.

  15. Kt Says:

    Hi Sally,

    The dark patches could be bruising. I’ve gotten new goldfish before because of their interesting pattern only to have it fade within a week or two because it was actually bruising from being transported to the pet store.

  16. Amanda M Says:

    We got a new tank last year 80 gallon i think. The only fish left from our old tank was the algae eater. We have had NOThing but problems from this tank. We cant get the water to stay clear for anything, I have bought the expensive Ehium filter which doesnt use a filter only gravel and some other little things that come with them. We also got a UV light to help. This is the first week in over a year that its staying clear.The algae eatter finally died. I have treated the tank and the fish all with fungus cause all the fish that weve gotten have died.
    Last week I drained the whole tank, cleaned everything and put it back together. the water is actually clear and pretty. The water still smells. Which ive been told fish water shouldnt smell any? Now i have one of my goldfish with black spots and is lifeless just laying around. I tested for ammonia and a 5 in 1 to see what was off. Ammonia is perfect, Nitrates and ph are off just a tad. What could be wrong with the tank? Lost and about to give up. I love my tank cause it is pretty but I cant keep having $80-$90 sewer and water bills. Help please!!!!

  17. Stacy B Says:

    I have a 30 gallon tank, 4 fancy goldfish (various sizes) and few regular ones… there is one of the regular ones that I think is picking on the larger fish. It looks like it could be tail rot, but no redmarks? One completely lost his fin on one side. Will that recover if I separate them? They all mostly seem to have their spunk to them, just look beat to heck. Thanks for you help!!!

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