Formula for SG

kospaintball · 01-21-2009, 02:55 PM

Is there a formula for how much Freshwater you would have to add to your SW tank in order to lower the SG from...pfff i dunno lets say 1.022 to 1.021?

I don't have an ATO set up yet and i add FW a little Every day to keep water at the same level, and i was wondering how much of a jump that is in the SG

BigAl07 · 01-21-2009, 02:58 PM

Thats not a HUGE jump if it's ok a 24 hour period. Daily top off (unless it's extreme) shouldn't make a HUGE change but the problem with DAILY is if you skip a day (then 2 then 3) and THEN try to top off you could shock them live-stock.

chipmunkofdoom2 · 01-21-2009, 03:26 PM

I was never good at math, but I think I figured out a pretty solid formula a while ago when my salinity for water changes was always too high.

If you have one gallon of salt water that you mixed too high at 1.030 and you wanted to get the salinity down, if you add one gallon of fresh water, you get 1.015. The long way to do it is to sit and reason your way through, but the easy way is to do it like averages on tests or something like that. Add all the numbers together then divide by the total number of gallons. Like, 2 gallons of salt water plus one gallon of fresh water:

30+30+0/3 = 1.020

I only work with the hundredths and thousands because the numbers before that generally don't change, but I think you can upscale that formula as much as you'd like.

3 gallons of salt + 1 fresh:

30+30+30+0/4 = 1.0225

4 salt, 1 fresh:

30+30+30+30+0/5 = 1.024

5 salt, 1 fresh:

30+30+30+30+30+0/6=1.025

You could easily apply this with any amount of fresh water added to any amount of salt water at any specific gravity. Use this formula

(G*S)/(G+A)

G is the amount of saltwater (doesn't matter what unit, gallons, liters, ounces, haha) and S is the specific gravity (remember to only use the hundreds and thousands). Then you divide by G, the amount of saltwater, plus A, which is the amount of fresh water added. Make sure that the A you're adding is the same unit as G (such as gallons) and be sure that the G is your TOTAL water volume. Tank, sump, fuge and all.
For example, if your tank water is at 1.025 and you have a total of 75 gallons of tank water and you dump an ENTIRE gallon of fresh water in:

(75*25)/(75+1)= 1.02467

Again, I'm not sure if this formula is right because I thought it up myself

But it works for my water changes and all and I don't see any reason why it can't be scaled up to work for an entire system.

chipmunkofdoom2 · 01-21-2009, 03:33 PM

If we felt like doing the algebra for your tank, which is 14 gallons, we could find out EXACTLY how much water it would take to lower the salinity from 1.022 to 1.021.

(14*22)/(14+A)=21

14*22 = 21(14+A)
308 = 294 + 21A
14 = 21A
A = .666(repeating)

So you would need to add two thirds of a gallon to drop 14 gallons of water from 1.022 to 1.021... I think. Then again, algebra was always my weak spot. Have someone who can actually do the math check my math

01-21-2009, 02:55 PM	#1 (permalink)
kospaintball Scopas Tang Join Date: Dec 2008 Posts: 453 View Gallery Karma Komment	Formula for SG Is there a formula for how much Freshwater you would have to add to your SW tank in order to lower the SG from...pfff i dunno lets say 1.022 to 1.021? I don't have an ATO set up yet and i add FW a little Every day to keep water at the same level, and i was wondering how much of a jump that is in the SG __________________ 14 gal nano 1 maroon, 1 clown goby, metalic green hairy mushroom, trumpets, Devil's hand leather, Frog spawn, kenya tree short tentacled plate, cinularis tree, zoas, clove coral

01-21-2009, 02:58 PM	#2 (permalink)
BigAl07 Do I look as lost as I am Join Date: Mar 2007 Location: Waynesville, North Carolina Posts: 21,549 View Gallery Karma Komment Add yours!	Re: Formula for SG Thats not a HUGE jump if it's ok a 24 hour period. Daily top off (unless it's extreme) shouldn't make a HUGE change but the problem with DAILY is if you skip a day (then 2 then 3) and THEN try to top off you could shock them live-stock. __________________ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~ *Live Rock Rubble will do the SAME thing as Bio-Balls and is NOT a suitable replacement for BIO-BALLS in a Reef System! It's ALL gotta go!!* Nitrate (NO3) reduction is directly proportional to percentage of Water Change. Allen's home-made formula...currentNO3-((%WC.01)currentNO3)=finalNO3 (thanks Luukosian) This means if you change 50% of your total water volume (That's EVERYTHING) you'll get a net reduction of (NO3) somewhere around 50%. Ask me about how to increase your REEF budget without going without FOOD!!* Big Al's 10g Julie's (BigAl's Gal) 6g NanoCube Gone but not forgotten BigAl's Slow 90g Tank Chronicle Allens OFFICE 12g Nano-Reef

01-21-2009, 03:26 PM	#3 (permalink)
chipmunkofdoom2 Reef Lobster Join Date: Nov 2008 Location: Stewartstown, PA Posts: 1,305 View Gallery Karma Komment	Re: Formula for SG I was never good at math, but I think I figured out a pretty solid formula a while ago when my salinity for water changes was always too high. If you have one gallon of salt water that you mixed too high at 1.030 and you wanted to get the salinity down, if you add one gallon of fresh water, you get 1.015. The long way to do it is to sit and reason your way through, but the easy way is to do it like averages on tests or something like that. Add all the numbers together then divide by the total number of gallons. Like, 2 gallons of salt water plus one gallon of fresh water: 30+30+0/3 = 1.020 I only work with the hundredths and thousands because the numbers before that generally don't change, but I think you can upscale that formula as much as you'd like. 3 gallons of salt + 1 fresh: 30+30+30+0/4 = 1.0225 4 salt, 1 fresh: 30+30+30+30+0/5 = 1.024 5 salt, 1 fresh: 30+30+30+30+30+0/6=1.025 You could easily apply this with any amount of fresh water added to any amount of salt water at any specific gravity. Use this formula (GS)/(G+A) G is the amount of saltwater (doesn't matter what unit, gallons, liters, ounces, haha) and S is the specific gravity (remember to only use the hundreds and thousands). Then you divide by G, the amount of saltwater, plus A, which is the amount of fresh water added. Make sure that the A you're adding is the same unit as G (such as gallons) and be sure that the G is your TOTAL water volume. Tank, sump, fuge and all. For example, if your tank water is at 1.025 and you have a total of 75 gallons of tank water and you dump an ENTIRE gallon of fresh water in: (7525)/(75+1)= 1.02467 Again, I'm not sure if this formula is right because I thought it up myself But it works for my water changes and all and I don't see any reason why it can't be scaled up to work for an entire system. __________________ Will sell my body for ricordea DIY High Power LED Retro!

01-21-2009, 03:33 PM	#4 (permalink)
chipmunkofdoom2 Reef Lobster Join Date: Nov 2008 Location: Stewartstown, PA Posts: 1,305 View Gallery Karma Komment	Re: Formula for SG If we felt like doing the algebra for your tank, which is 14 gallons, we could find out EXACTLY how much water it would take to lower the salinity from 1.022 to 1.021. (1422)/(14+A)=21 1422 = 21(14+A) 308 = 294 + 21A 14 = 21A A = .666(repeating) So you would need to add two thirds of a gallon to drop 14 gallons of water from 1.022 to 1.021... I think. Then again, algebra was always my weak spot. Have someone who can actually do the math check my math __________________ Will sell my body for ricordea DIY High Power LED Retro!

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