Q: AB wrote,

Your site is WONDERFUL, soooo much info. My question is how do you calculate the different water conditions? I changed a 10 gal community [tank] this past Sunday & have lost two female bettas & 2 little orange school fish. I’m not good with figuring the math part like chlorine drops, StressCoat and salt for different size tanks. Who could I put with one male swordtail & one very black adorable molly?

A:Hi. Thanks for writing in. The question of calculating water additives like dechlorinator is a good one. This confuses a lot of people. Measuring accurately is important but more-so for medications and salt then it is for water conditioners. Dechlorinators are pretty safe and as long as you get the amount relatively close to what is suggested on the bottle youll be fine. For water conditions like StressCoat, AmQuel & NovAqua or Prime, for example, just follow the directions on the bottle.

Lets take StressCoat as our example. The directions say: To remove chlorine and neutralize chloramine–add one teaspoon for each ten gallons of tap water.

Since there isnt a serious risk of overdose associated with StressCoat we dont have to worry about calculating displaced water lost by adding sand and tank decorations. We can just assume there is about 10 gallons in your 10 gal aquarium. (Even though in reality, there is probably only about 8 gallons or so).

So if you are changing all the water in your 10 gal then you need the full one-teaspoon as directed. If you are changing half the water then you need half of one teaspoon (1/2 tsp.) If you are only doing a 25% water change (or 1/4 of the water) then you only need 1/4 tsp of StressCoat. I suggest treating the tap water in a clean bucket and letting it sit for a few minutes outside the aquarium so the dechlorinator has a chance to work. Some aquarists like to wait a lot longer but I never have any problems with an immediate change. I use one of those 5 gal painters buckets from The Home Depot. Since I know it holds 5 gallons, I fill it up and treat it with 1/2 tsp of dechlorinator. If you use another brand like Prime, which is more concentrated, the dosing may be different.

When treating with salt, you will also want to treat only the new water in a separate bucket. Salt doesnt evaporate so if you just keep
adding doses for 10 gallons directly to your tank, very soon it will become so concentrated that it can seriously hurt or kill your fish. So if you are siphoning out 5 gallons of water, only add enough salt to treat the replacement 5 gallons. Dont add enough salt to treat the entire 10 gallons or it will mix with the remaining salt (5 gallons) which will leave you with enough salt to treat 15 gallons worth of water in only a 10 gallon tank. Otherwise you have 50% too much salt. Also, remember some species dont handle added salt well at all. While Mollies thrive with a little salt, your orange fish may not. Bettas also dont handle salt very well, though small amounts to treat certain illnesses can be beneficial.

More Helpful Info:

Equation to determine actual U.S. gallons of your aquarium.
Measure in inches, the Length x Width x Height and divide by 231.
We’ll call the cubic inches of the aquarium “a”.

L x W x H = a

a ÷ 231 = X

X = the amount of water your tank really holds before adding substrate, fish and a filter.

L=20″ x W=10″ x H=12.5″
20x10x12.5 = 2500

2500 ÷ 231 = 10.82 US gal. (So my 10 gal tank really holds 10.82 gal when filled up)

Then do the exact same thing again but this time only measure the Length x Width x Height of your aquarium substrate. This method isn’t completely accurate as substrate particals differ in size (gravel, sand, stones, etc) and open space between granuals differ, but it’s close enough for medicinal purposes. We’ll call the cubic inches of the substate “s”.

L x W x H = s

s ÷ 231 = Y

Then subtract Y from X to get a good estimate of the amount of water your aquarium is actually holding.
X – Y = true water volume

L=20″ x W=10″ x H=2.75″
20x10x2.75 = 550

550 ÷ 231 = 2.38 US gal. (my aquarium is holding 2.38 gal of sand)

10.82 gal – 2.38 gal = 8.4 US gal

My aquarium holds about 8.4 gal of water after adding substrate.

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Reader Interactions


  1. Christie says:

    Dear Aussie’s, Canadians, and all the rest from across the Pond:
    I know we get a TON of international readers to the Nippyfish Blog so don’t pay any attention to that American gibberish I just wrote about regarding “inches” and “US gallons”. We like to make things difficult on ourselves… It’s the American way. (see news headlines if you need proof). 😉
    Thanks… and of course, I will continue to try and incorporate metric conversions whenever possible in my postings.

  2. Anonymous says:

    i had Betta when i was young and it lived for a long time. I am going to buy another what should i do to reset up the fishbowl, what temperature should it be at, what ph level, how long should i let the tank set before buying a fish, what food should i buy and what cleaners do i need to get?
    please help…. thanks

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