Need help, kind of urgent...


Pygmy Octopus
My baby octo passed away two days ago and the reason seems to be the salinity. I tested all the water parameters and they're fine but my salinity seems to be off the chart(high). I have a hydromometer and the needle points to the very top.

What could cause this? I buy pre mixed ro/di saltwater from a lfs and the salinity in their water is perfect. Maybe i got a bad batch? Is there a quick fix/easy way to bring the salinity down and keep it stable?


Pygmy Octopus
I just found this in another post by Andy Lister:
"Dont forget to alter the temp of your RO/DI water to your tank temp before you make it up as this will affect the salinity!

Check the tank salinity too before adding it... if you constantly add water that is about 36ppt to a tank and never check the tank you will be surprised how quickly it shoots up with evaporation!"

Now, I have two fans blowing, one on my sump and one on the top of my tank. How can i control the salinity, by both bringing it down quickly and keeping it stable?


You will have to do a partial water change...but don't your high sal water in a large container ( we use a 50 gallon plastic garbage great) and add in lower sal water gradually, until you get to 1.026...the high sal water can have fresh water added to it to bring it to the same level, and then you can use it for water changes.
This is why we advocate mixing/obtaining water for changes up more than 24 hours prior to use, so you can check the salinity.
Sorry about your octo... :(


Pygmy Octopus
thanks greg. but i'm still unsure about how to maintain the salinity since i have a lot of evaporation? I add about 1-2 gallons of the pre-mixed water i get from my lfs every day. Is evaporation the main cause of high salinity?

Also, how much of a water change will i have to do for a 40 gal. to get the salinity back down?


My apologies if I'm stating things you already know, I really don't mean to be lecturing you needlessly, but since I'm not sure what you know and it sounds like you might have missed a couple of these points:

Salinity is a measure of how much salt is dissolved in the water. When the water evaporates, the salt doesn't evaporate as well--it stays behind. Because of this, any container of saltwater will become more and more concentrated as the water dries up.

We know that every tank evaporates at least somewhat--yours, with fans, will evaporate rapidly. We know that the ONLY thing lost when evaporation occurs is water. Thus, if we want to put things back to the way they were when we started (same amount of water, same salinity,) what should we add to the tank? The exact same thing we had lost--water, and only water. Thus, adding pre-mixed saltwater to make up for evaporative loss only adds salt to the tank. Another way to think of it is to say that your tank currently has a pound of salt in it. With each saltwater makeup, you add another few ounces of salt. Before too long you suddenly find you have two pounds of salt dissolved in your tank instead of just one--now your salt concentration per gallon of water is much higher. Regardless of how much water is present, the amount of salt in the tank remains the same unless you physically remove it by removing saltwater in buckets.

As Greg said, to lower the tank's salinity, you need to remove salt by taking some saltwater out and replacing it with freshwater. To raise it, you would do pretty much what you've been doing--replace evaporation loss (which removes only freshwater from the tank) with saltwater. If you needed to raise salinity in a hurry, you would make an extra-strong batch of saltwater to add to the tank.

Follow Greg's advice to get your salinity back to normal--take out a gallon of saltwater, add a gallon of freshwater, let it mix (it will take time, probably 15 minutes or so, for the sump and tank to mix thoroughly) and check your salinity. If it's still too high, repeat, until it's at the right level. Once you've established the right salinity for the tank, from then on add ONLY freshwater to replace evaporative loss. You'll keep monitoring the salinity on a regular basis and make minor changes (by adding or removing saltwater) as needed.

Keep in mind that replacing evaporation is not a "water change." What we mean by water change is discarding roughly 10% of the tank's volume every week or so. In a 100-gallon tank, this means pulling 10 gallons out in buckets (to be dumped down the drain) and replacing them with 10 gallons of newly-made saltwater (since we just dumped saltwater, now we need to replace the same amount of salt we just took out.) We do this because over time, certain waste products from all the critters and bacteria in your tank build up--most of them don't evaporate either. Water changes assure that we remove those wastes that can't be removed by filtration devices.

1-2 gallons of evaporative loss per day sounds a little high to me, but then everyone's conditions vary. Why do you have the fans? Is it to keep temperature down? Not using them would be one way to slow down evaporation. Another way is to keep the lid on your tank and sump. Both of these will raise the temperature of the tank at least a little (evaporation is one of your tank's main means of cooling,) so if your tank is too warm, you'll just have to keep the fans and accept replacing 1-2 gallons a day, or look into purchasing a tank chiller (expensive.)

Hope this helps!


Pygmy Octopus
rrtanton, thank you very much for the info. I actually just got back from the lfs and i'm kicking myself in the butt because I feel embarassed that i didn't realize the problem. Because of you guys and my lfs i completely understand how to fix the problem, so i just picked up ten gallons of ro freshwater to add to my tank gradually. Thank you very much for your help guys.


Staff member
Sorry about your octo. There's a lot to learn about keeping a salt water aquarium and it's not unusual to have one problem or another like this.

I too used a fan with my bimac (don't need to keep the temperature so low since no octo is in the tank right now). I have to replace about 1/4 to 1/2 gallon a day, and started using one gallon jugs of distilled water. I can pour directly from the jug into the sump. Works very well!


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