Jump to content

Water changes with only cold tap water available

Recommended Posts


Does anyone know the ratio for mixing boiled water with cold tap water in a 5 gallon pail to refill an aquarium for 74°f or how I could figure it out? My house uses a water softener and the only taps that aren't on it are the ones that would have a garden hose attached to it. The closest one to the aquarium is in my garage. The temp out of the tap is around 45°f I think, or whatever is equivalent to 10°c. My kettle does 60oz.

Link to comment
Share on other sites

I think it'll take some experimenting. depending on how big a water change in total gallons you are doing you could run the hose to a large bucket inside then let the water come to room temp first. That may be close enough to do a small water change without dropping the temp in the aquarium too much.

Link to comment
Share on other sites

Just get one of those $15 infrared temp guns, easy peasy.

I only have one 29 gal going right now, and I do weekly 50% changes.

I have a 20 gallon trash can, and I fill it with 15 gals from 3x5 gal buckets of cold.

I have a big steel cooking pot I keep for fish water and botanicals, and I fill that with cold and pop it on the stove. 

For all changes I’m aiming for 76F, but in summer I can get that with 90% of a boiling pot and in winter it takes the whole pot plus 2 kettles because the groundwater is so cold (still city water).

I mix in place, dechlor, and pump into the tank with an electric pump.

I use another pump and another line to drain.

All gets stored in the can when not in use.





Edited by AdamTill
Link to comment
Share on other sites

I heat a quart of water in the microwave to just under too hot to touch, and mix with approx 1.5-2 gallons of room temp water in a bucket, using my hand to measure against the tank water to ensure its the same. You'd be surprised how accurate it is. 

  • Like 1
Link to comment
Share on other sites

You can use the equation:

q = m * Cp * (Th - Tc)


 q is heat energy 

m is mass

Cp is specific heat capacity 

Th and Tc are the hot and cold temperatures 

The energy given ny the boiling water to the cold water should be equal, so

q1 = q2


mc * Cp * (74F - Tc) = mh * Cp * (212F - 74F)


mc * (74F - Tc) = mh * 138F

Multiply through by the density of water and convert the mass to volume, so

Vh = Vc * (74F - Tc) / 138F

Where Vc and Vh are the volumes of your cold and boiling hot water and Tc is the temperature of your cold water in F. 

This takes some assumptions, but should get you within a couple of degrees. 

How this helps and wasn't too far Dien the science hole.


  • Like 2
  • Thanks 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...