How Much water should I change per week?

  • #1
Hey all,

Have had a fish tank setup for about 6 months. How much water do you think I should be changing weekly? I have a 20 gallon tank and have been doing two 5% changes per week. So a total of 10% water changes. The fish I have I have in my tank are:

1 - Golden Panda Molly
1 - Creamsicle Molly
1 - GBR
1 - Upside down catfish
1 - Cobra Guppy
1 - D. Gourami
1 - Mickey Mouse Platy
1 - Balloon Belly Molly

I am mainly concerned about keeping high water quality due to the fact I have a GBR and I heard they are very sensitive to water quality. My tap water has a little over 1 PPM ammonia and has a PH of 7.4. My tank has a 0 PPM ammonia and a PH of 6.6. I was doing small bi-weekly water changes because I did not want to increase the ammonia or increase the PH in my tank too greatly and kill my GBR. It seems to have been working well so far. All the fish seem happy and thriving. GBR has been in the tank for 3 weeks now.

One of my major concerns is now with nitrates. I am using the API liquid test kit. For the longest time I kept get 0 PPM over and over for the nitrate test. I was like great I never have nitrates! I then read somewhere that you need to shake the API nitrate bottle really well to get an accurate reading. I did this and nitrates were around 30 PPM after a small water change! I am concerned about the nitrates being too high for the GBR.

Anyway, wanted to get peoples thoughts on maintaining good water quality for the fish especially the GBR since it is so sensitive. I want to be able to keep nitrates consistently below 20 ppm but also afraid of doing large water changes that would could spike PH and add small amounts of ammonia. Like a lot of things in the fish tank world I am sure I will need to find a happy medium to accomplish this.



  • #2
Your tank is constantly making nitrates so 30 ppm is not the end of the world. You seem to be doing just fine as far as water quality. I am more interested in why your tap water ph and tank ph are so different. Have you ever checked your tanks KH? KH is what keeps your PH stable. I wonder if your tank has very little. As your fish add wast to the tank the PH will get lower if you have no buffering capacity and this is what leads to PH crashes.

  • #4
What filter are you using? If you could use seachem matrix it is suppose to grow bb that eats nitrates. As for ammonia in your tap, seachem prime should make it safe while your bb eats it.
As far as I know (not very far really) the BB that eats nitrates can't grow in an oxygen rich environment so I think he was just doing the test wrong.
  • #5
My 3 tanks consistently show 20-40 ppm nitrates, my tap water = 20 ppm. Currently all I have are bristlenose and Colombia zebra catfish but they're all doing fine, as are the plants. The BNs have spawned twice and babies also all doing fine. The 45g has 7 adult BNs and about 50 babies so I do water changes every day, along with substrate vacuuming. Once the babies are gone I won't have to do this as often. The nitrates I figure are just a fact of life here.
  • #6
you could try adding a few live plants to help get nitrates down a little.
it has been said that they also help reduce ammonia too.
I would try a couple of tiger lotus bulbs in the rear of the tank (they can grow large if you let them but my red tiger lotus even survived being pruned back to the bulb). they are said to consume a lot of nutrients.
also, changing 15-20% of your water at a time should not harm things either.
I have a very heavily stocked well planted 20 gallon and my nitrates typically stay around 5-10ppm. sometimes going up to 20, but never in to the "red" on the API test.

  • #8
it depends on how much you stock and how much nitrates you have. I do 25% water changes every other week.

I say 90% water changes being part of the goldfish community and all
While 90% is good for a major tank emergency (like once someone spilled coffie into their tank here, another spilled a test kit in, another lost some pasta thing into the tank, another had a kid pour food into the tank, another added too much ammonia to a tank for fishless cycling and added it to the wrong tank, which had fish, list goes on and on) That big of a water change is very likely to shock the fish since its more then needed.

EDIT: I also use And my nitrates are at zero most the time, I let it go up for my plants once in awhile. But water changes are good for other reasons too.
  • #9
I also like to have a high weekly turnover of water, usually divided into 2-3 times a week, totally around 90%.
As you have a GBR, I would strive to keep the nitrates well below 20 ppm.
When you tested your tap water, did you first allow it to gas out (sit, aerated for 24 hours) to allow dissolved gases to disperse? It's really the only way to get an accurate read on your tap water parameters.
All the best, rick

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