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120 reef 40g trigger emerald sump kessil ap700 light 6ish fish
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Discussion Starter · #1 ·
ok... probably a silly question... but the specific gravity of my tank is 1.027ish (swing arm hydrometer) I know they are inaccurate but its all I have atm... would it be safe to do a 5g water change with water that is a few points lower? 46 gal tank and stock is shrimp, hermits, few snails, and 4 fish... also... had a turbo kick the bucket and didn't remove it right away... no ammonia spike that I seen... but nitrates are a bit up... could I do multiple changes this weekend to lower the salinity some more and try to knock out the nitrates... usually do 1 5g change weekly... probably silly questions... thanks
 

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120 reef 40g trigger emerald sump kessil ap700 light 6ish fish
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588 Posts
Discussion Starter · #3 ·
Would have to replace it with either distilled or treated tap... don't have a rodi and no one sells any... and a few gallons a day? That seems like alot... and no sump so it would be added direct to the tank...
 

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The only silly questions are the ones that aren't asked.
You can mathematically determine how much to change if you know the volume of water.
For example, if you actually had 46g in the tank, and if you want to lower it to the standard reef s.g. of 1.026 when you are at 1.027, then divide 26 by 27 and multiply by 46 and you get 44.3 (rounded off) so if you remove 1.7 gallons of the water at 1.027 and top up with 1.7g of unsalted water then you will have 46g of water now at 1.026.
If you wanted to use a lower salinity water to replace what comes out, you would have to use a larger volume. I don't know how to write algabraic terms on this but I calculated you would need to remove 3.3g of 1.027 and replace with 3.3g of 1.013 or 6.6g out and in with 6.6g of 1.020.
It really doesn't matter if you do the first method with just water, or the method using lower salinity water because you are going to end up with the s.g. down one point both ways.
Going down one point is NOT going to affect the tank inhabitants even when done fairly quickly, but if you are concerned then just add the unsalted water a bit at a time.
If you want to drop to a lower s.g. than 1.026 (but I don't know why) then it would be prudent to do it in stages probably not more than .002 each time.
However, your tank while rated at 46g might be slightly less, and then too, you have some of the volume taken up with equipment, probably live rock, maybe corals, etc.... so actual volume might be anywhere between maybe 30g and 40g. This means actual volume changed will be less than if the tank really contained 46g.
If you can reasonably estimate actual volume you can plug that in for the 46 in the top calculation, and if you want something different than the 1.026, then just put in the last two digits in place the 26 where I used the 26 divided by 27.
As for the nitrates, I would lower the s.g. to what you want using unsalted water, and then change out water with new salt water of the same s.g..
If you have an actual 40g of water and nitrates are at 25, changing 5g would lower the nitrates to 35 divided by 40 times 25 equals 21.875.
If you change 10g of a 40g total then 30/40 X 25 = 18.75.
 
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