Changes in Salinity?

[MSU Fan]

I bought a floating hydrometer when I started my tank (I am going to purchase a refractometer when I get the $$$). When I was pre-mixing my SW to cycle my tank with (using cured LR), the salinity showed at 1.024 or so on the Hydrometer. I just rechecked it (5 days later), and the salinity shows 1.018 or so - what does that mean for the life in my tank???? How should I raise back up, or do I need to before my cycle is over? TIA

 

[Marc118]

How long was the salt mixing before you tested it? You could have just had the hydrometer in a concentrated area of the water before it was all mixed. The SG will raise as water evaporates over the next few days, then you can add a diluted mix of salt and RO until its at the desired SG. After that you will want to top off with just RO.

 

[MSU Fan]

Will the lower SG affect the cycle (such as cause more die off??)?

 

[Marc118]

Im not sure, but you wont have to worry about it IMO. As long as you top off with a mix of salt to keep the salinity up after evaporation you should have no problems with the cycle. BW uses an SG of around that I believe and beneficial bacteria lives in that no problem. I guess we will both learn something if someone chimes in here.

 

[MSU Fan]

What is "BW"?

 

[Marc118]

Brackish

 

[Kurt_Nelson]

Your low SG shouldn't really effect your cycle. As mentioned earlier, just top off with saltwater instead of freshwater, and you'll slowly climb higher. But if I remember right, you're cycling a fairly small tank. So... if you really wanted to be safe, you could do a 50% water change using something like 1.031 water, and that should bring you back to around 1.024. You're early enough in your cycle that it won't make much of a difference.

Just make sure you let the just-mixed saltwater "rest" a day or two before you take a SG reading. Until it rests a bit, the salinity and pH readings will be off. However, I wouldn't expect it to be off by as much as you saw.

I've only used a swing arm once, but if I remember correctly I think they're calibrated for a certain temperature water. Was the first sample you took a reading from at the same temp as the second sample? If not, did you apply a correction factor at all? Could that be the issue?

In the big scope of things, a refractometer is probably one of the best investments you can make.

 

[MSU Fan]

Thanks for the info - actually, when we mixed the SW last week, we still had ~10g of SW leftover, which has been circulating at ~78 degrees since last Saturday. I am stupid, and didn't bother to test the salinity of that water yet (something I plan to rectify tonight), which may explain some things...

Also, this is a floating glass hydrometer, not a swing arm (although I am not sure they are much better...).

I am going to retest the SG tonight in the tank and in the holding tub and see if there is a difference. I will re-calibrate the holding tank to 1.031 and then mix that for a day, retest it to verify that it is still at that SG, then pwc 50% to achieve optimal SG.

Also, when premixing, would it be better to add the salt and water and heater all at the same time, or preheat the water in the tub, then later add the salt and mix?

I guess if it was easy, everyone would be doin' it!

 

[Kurt_Nelson]

I've used a hydrometer with beer making, and those definitely need a correction factor at different temps. But again, it shouldn't make the difference you're seeing and it sounds like you're on the right trail with the "unknown" 10g of water you used.

Regarding mixing SW, I fill up a bucket with 3.5 gallons of water, and add 2 cups salt mix (Reef Crystals), and stir the heck out of it for a minute or so. The water is probably about 50 deg F, and the salt dissolves fine. I then dump it into my holding tub and bring it up to temperature. I make SW up in 3.5 gallon batches as it keeps it simple - I only have to remember one set of numbers! I've also got a MaxiJet900 in there keeping things stirred up and aereated. After a couple days, I check the salinity and adjust if necessary.

I used to preheat the water and mix in the tub. It seemed like I couldn't really stir up the water good enough to dissolve all the salt mix, so that's when I switched to just mixing the salt in the bucket with the cold water. For me, it's easier and actually seems to mix the stuff better.

 

[roka64]

If you are cycling your tank, I would not da a PWC, unless needed. You can just bring up your salinity as you add water. I premix my water the same way Kurt does.
I add the heater, ph salt and water at the same time. I have also found the hydrometers don't give accurate readings when the water is still cold.

 

[MSU Fan]

I just tested the water in the tank and holding tub - the tank is still ~1.018 SG, and the holding tub (which hasn't been moved or added to since we premixed last saturday (it has been mixing since then, just nothing has been added to it) is still at ~1.023. Very odd.

 

[Marc118]

Same temps?

 

[MSU Fan]

within 1 degree
Holding tank~78
Display tank~79

 

[roka64]

That is strange. I would just start doing top offs with the premixed SW until you reach your desired salinity.

 

[MSU Fan]

That's the plan at this point - I also noticed that the pH keeps dropping lower and lower in the display tank - started right at 8.2 - now it's down to ~7.6 -- something else I should be addressing?

 

[roka64]

That's the plan at this point - I also noticed that the pH keeps dropping lower and lower in the display tank - started right at 8.2 - now it's down to ~7.6 -- something else I should be addressing?

Not during your cycle, it may even take a few months to balance out.

 

[Ziggy953]

True...your PH will drop down until your tank stablizes, after a couple months. I also mix my SW in a way similar to that of the other guys that chimed in. It is always a good idea to mix your SW at least 24 hours before use. Your hydrometer is not going to give you accurate readings until the water is 78-80 degrees. Sounds like you have a plan on correcting your SG so I will leave it at that.

 

[MSU Fan]

Thanks again everyone - I (and my tank) really appreciate your insights and patience!