What Causes an SG DROP

[jonboyb]

My tank has been up for a couple weeks now and I did a 35% PWC a couple days ago after the cycle. It's a 28 gallon with 40 lbs LR (mix of fully cured and pre-cured) and 10 lbs sand. Maybe 3 lbs of LR rubble in rear sump filter basket. No other filters.

Twice now I've had my SG actually drop using a refractometer and checking it also with a hydrometer because I couldn't believe it. When I first started it up, I had an SG=1.024 that ended up around 1.020 after 2 weeks. Now 2 days after my PWC SG has dropped from 1.023 to 1.022. I have not added any additional water to account for evap in either case.

Is my LR and associated growths/hikers pulling enough trace elements from the water to actually lower my SG. I'm using SeaChem Reef Salt if that matters. I expected SG to always rise due to evap, never drop??? I was going to introduce some cleaner crew in the next few days, but now I fear the SG drop might stun the crabs and shrimp.

 

[Kurt_Nelson]

Are you actually reporting the readings from your tank, or are those the readings of the salt water you're putting in the tank?

How soon after mixing the salt mix into the water are you taking your reading? Freshly mixed salt water - in my experience - will often read high, versus taking the reading 24 hours later.

Other than measurement error, I don't know of any reason SG should drop over time.

 

[melosu58]

Are you measuring your PWC water and tank water to make sure they are the same before adding?

 

[fijiwigi]

I experienced similar issues until I bought a refractometer I found my actual SG was way different than what my hyrdromoter was leading me to believe.

 

[ccCapt]

I agree with Kurt. Has to be a testing error. Nothing short of adding fresh water to the tank will lower the salinity any measurable amount.