How much does evaporation change the salinity of a tank? I have a friend who tops off his tank with fresh water between his water changes every 2 weeks. Is this sufficient enough to keep the salinity in a stable range?
Salinity and Evaporation
- Thread starter TMountains16
- Start date
- Tagged users None
Salinity won't rise unless he doesn't top off. Water evaporates, the salt does not, so by keeping the water at good levels, the SG won't rise on you.
Yuki Rihwa
Well-Known Member
- Joined
- Jul 16, 2017
- Messages
- 751
- Reaction score
- 853
I don't use ATO and my tank is none cover lid top, I just do manually fill RO water 1/3 gallon in the morning and 1/3 at night after I came home from work to keep my salinity stay pretty much exact 1.025 all time, if I for got either 1 of my fill up schedule the salinity would be go up to 1.026 which still OK but I don't want to miss it, it's my hard job while at home 
Edit
If I missed my schedule my salinity would rise LOL
@Florida Sunshine Thanks for correction LOL My brain seem to still in sleep mode
Edit
If I missed my schedule my salinity would rise LOL
@Florida Sunshine Thanks for correction LOL My brain seem to still in sleep mode
Last edited:
Florida Sunshine
Well-Known Member
Actually if you missed a top off your salinity would rise.
As to the original question, it depends on the water volume in the system. If you have a small volume of water in the tank then even minor evaporation can change your salinity. Amount of humidity and temperature will also effect your rate of evaporation so there is really not a "right" answer.
Small, frequent top-offs are preferred which is why many of us have auto top offs that replenish a little bit of water at a time to compensate for the evaporation.
As to the original question, it depends on the water volume in the system. If you have a small volume of water in the tank then even minor evaporation can change your salinity. Amount of humidity and temperature will also effect your rate of evaporation so there is really not a "right" answer.
Small, frequent top-offs are preferred which is why many of us have auto top offs that replenish a little bit of water at a time to compensate for the evaporation.
Greybeard
2500 Club Member
The right answer is... It depends.
A tank with no sump, a glass lid, and cool LED lighting, in a high humidity area, is going to evaporate very little. A surface swirling, open top system with a couple metal halides and a sump with a lot of splash, in a dry environment, is going to evaporate a lot.
Not hard to figure out how much your system is evaporating... mark the tank (or sump), let it go 2 days, and see how much water it takes to refill to the line. Once you know how much you're loosing, you can use that, along with the total volume of your system, and run it through a calculator like this one:
https://www.hamzasreef.com/Contents/Calculators/ResultingSalinity.php
Salinity is the easiest parameter to keep rock solid. I can't see _any_ benefit in allowing it to fluctuate daily. A safe DIY ATO system can be assembled very inexpensively.
Randy Holmes-Farley
Reef Chemist
Staff member
Super Moderator
Excellence Award
Expert Contributor
Article Contributor
R2R Research
My Tank Thread
If the tank water volume decreases by 10% (2 inches in a 20 inch tall tank), the salinity rises by 11.1%, so 35 ppt becomes 38.9 ppt (sg = 1.026 becomes 1.0289), which IMO is too big of a change unless the water is added back very slowly.
It also changes everything in it by 111.1%, so calcium at 420 ppm becomes 467 ppm, etc.
It also changes everything in it by 111.1%, so calcium at 420 ppm becomes 467 ppm, etc.
Dave_buffalo
New Member
I use the above calculator for water changes when my salinity creeps low, and it seems reliable. But the "Water Evaporated" calculation is totally incorrect.
Start with: 55 gallon tank at 1.024 PPT. Evaporate 5 gals and it says resulting salinity is 1.126 PPT.
Unless I'm reading it wrong.
Attachments
reef_1
Active Member
I have a small QT with only Xenia, GSP and 5 snails and I fully neglected it recently, salinity went above 53ppt/1.40 due to evaporation.
I then brought it down to 1.027 in 30 minutes and everything survived even snails.
Ofc other inverts/corals likely wouldnt survive this, so this is not the suggested thing to do.
Randy Holmes-Farley
Reef Chemist
Staff member
Super Moderator
Excellence Award
Expert Contributor
Article Contributor
R2R Research
My Tank Thread
A 10% decrease in volume by evaporation (5 gallons in 50 gallons total volume) will give what you saw: 1.0240 --> 1.0267
KStatefan
2500 Club Member
The math in the calculator looks right
50 gallons/ 55 gallons =90.9% of the volume remaining 1.024 PPT/ 0.909 = 1.1265 PPT
Were you wanting to us specific gravity instead of PPT?
Randy Holmes-Farley
Reef Chemist
Staff member
Super Moderator
Excellence Award
Expert Contributor
Article Contributor
R2R Research
My Tank Thread
He entered sg values but had the ppt menu item chosen, Hence got the wrong answer.
When I got into the hobby I decided I didn't need an ato. I'd top off myself twice a day. But then life happens. You forget or get busy and suddenly a day or two later you're a few gallons down. So.... getting an ato is the best single thing I've done for my tank.
Randy Holmes-Farley
Reef Chemist
Staff member
Super Moderator
Excellence Award
Expert Contributor
Article Contributor
R2R Research
My Tank Thread
I’m having a hard time understanding what that means, and perhaps we do not disagree, but sg = 1.1265 is not the correct answer.
workhz
Valuable Member
10% evap is multiple by 1.11. It is 1.11 x .024 + 1. Not 1.11 x 1.024I’m having a hard time understanding what that means, and perhaps we do not disagree, but sg = 1.1265 is not the correct answer.
I think I was replying to someone that was multiply by the 1.024.
Randy Holmes-Farley
Reef Chemist
Staff member
Super Moderator
Excellence Award
Expert Contributor
Article Contributor
R2R Research
My Tank Thread
