W/C's and drop in SG

[Captainhemo]

Really give those hydrometeres a good couple of bumps and make sure it is sitting on a level countertoop when you read both when you are checking the tank and when checking you new water before adding it. Just holding the hydrometer at the slightest angle can "bind" the float/pointer and give you an incorrect reading.
As long as your nrew water is at or above the SG of the tank, there is NOWAY the SG can come out lower

[bv_reefer]

well i just put RO water, i didn't add salt because it was already at 1.023 so a 20% w/c with salt replacement would've kicked up to 1.025, i've never used saltwater as replacement water for a water change..

[Pescador]

A water change is different from topping up evaporation.
Top-up with fresh water because salt doesn't evaporate but water changes are done with pre-made salt water at the right SG and temperature.
You drain a % from the tank and add the same amount of new water.
Try to make any SG changes gradually.

[fishface]

get a refractometer. they're more expensive than hydrometer but one of the best investments for accuracy.

[Jason McK]

A 20% water change with out and salt is insane. If your hydrometer read 1.023 without adding any salt I would have thrown the Hydrometer in the garbage and stopped the water change.

[Aquattro]

you HAVE to add salt to your water change, that's the whole point!! You add water that is the same SG as the tank. And BTW, there is no way RO water was anything higher than SG of 1.000. That is pure water. Your equipment is either way off, or you're doing something terribly wrong...

[Aquattro]

ok, I gotta ask...is there anything alive in this tank??

[bv_reefer]

ya i've been looking into a used refractometer but honestly i don't think that SG has to be super accurate for coral health as long as it's consistent

[Jason McK]

I give up

[Rippin]

Originally Posted by Jason McK

I give up

What matters is that you tried....


Ok, here it is -
Regardless if you are using a hydrometer or a refractometer:

1. When doing a water change, test your aquarium water's SG, then mix the replacement fresh water with salt to match this SG number. It does not matter if the water you are mixing the salt with is RO/DI or tap water - it will not make a difference in your SG levels. If you do this your SG will not fluctuate (by anything significant) during a water change.

2. When doing a water top-ff due to evaporation, DO NOT mix salt water for this, just use straight RO/DI fresh water and top off to the SAME level in the aquarium before the evaporation.

3. An SG reading between 1.023 to 1.025 is good. I would consider 1.021 too low.