I let mine mix 3 days before usage, what do you all do and why?

It's recommended to let it sit at least 24 hours so the temp can stable and the salt completely mixes with either an aerator, or I put a power head in the bucket. Now myself, I usually mix first, let it sit for bout an hour or so , with a dechlorinater while I am siphoning the gravel. I can tell temp by my hand but then I have done this for many years. Some use a heater. Many people keep mixed water on hand just in case they have a problem that needs immediate attention with a water change.

Yep, I keep 30 gallons mixed all the time, at least...easier to do that way !

I let mine mix 3 days before usage, what do you all do and why?

Why do you let it sit 3 days?

I use water from my reef system for water changes in my ceph system. For the reef I keep 150 gallons mixed in a tub, and when its about 1/4 left, I refill it and add salt and let it mix, heat and aerate for 24 hours before using. I also test Mg, Ca, and Alk and adjust as needed. The 24 hours is mostly for heating. Keeping the water on hand has been very useful, so if you have the space, I say you do it! :grin:

I work at the University of Oregon, and my boss told me to do it that way. He is convinced that in the past using freshly mixed SW has killed Cuttlefish in the lab before. I just wanted to see if anyone had ever heard of that or are they being overly careful.

Hmmm... what kind of water are you using? Maybe the three days is to let the chlorine dissipate. I would recommend RO/DI water if you aren't already using it.

I work at the University of Oregon, and my boss told me to do it that way. He is convinced that in the past using freshly mixed SW has killed Cuttlefish in the lab before. I just wanted to see if anyone had ever heard of that or are they being overly careful.

Three days seems like overkill to me, 24 hours should be fine. There have been repots of freshly mixed saltwater doing damage to all kinds of animals, which is why the general recommendation is let it mix for 24 hours. Of course, matching parameters is also important - pH, alk, temp, salinity etc.

Were using "nano pure" which is distiled ran through ro di
Hmmm... what kind of water are you using? Maybe the three days is to let the chlorine dissipate. I would recommend RO/DI water if you aren't already using it.