Lowering SG

[Triggerjay]

Randy,
About a year ago I purchased a refractometer. I made the poor assumption that it was calibrated out of the box. It took a year to find out I was wrong. Yesterday I decided to check 0 with 0 TDS RO/DI. I was off by 5ppt. I calibrated the device to 0.

Checked my tank. 40ppt!!! I wanted to verify the result not knowing if it was a device error or if my tank was actually 40ppt. Off to the LFS I went. Tested device on their water, as well as testing my water on their refractometer.

Results say my refractometer is correct. My tank is 40ppt, and apparently has been for the last year.

I use a dosing pump to topoff with RO/DI. My plan is to increase topoff from 720ml/day (evap rate) to 1500ml/day. I will remove water via super wet skimming and manual removal. This will drop my SG very slowly.

Total system volume is about 70g.

I am assuming (got me in trouble last time) that alk, calcium, and mag will also drop and should be monitored frequently?

Over the last year, I have lost about 25% of purchased corals. The corals that make it generally do very well (must be very hardy). My tank is a mostly SPS mixed reef and I dose Prodibio and bionic two part and Kent tech-m mag. Mag rarely drops.

Tank lacks corraline algae, and has a hair algae issue. Don't know if these issues could be related. Guy at LFS said "is anything alive in that tank". Lol

 

[Sabellafella]

If your very worrydd about your sps and dont want to risk anything, i would find a salt mix that closely matches your parameters at a lower sg, then you could do wcs to lower your sg but also maintain somewhat near your current tank parameters. If you were to add rodi water to lower, your elements will drop with it

 

[Randy Holmes-Farley Verified Community Expert]

The total drop fro 40 to 35 ppt would drop calcium at 450 ppm to 394 ppm, and magnesium at 1350 ppm to 1181 ppm.

I do not know how big your tank is, so I cannot tell how long the sg lowering will take, but let's say 1 month as a guess?

So if you plan to add enough calcium and magnesium over the course of that month to offset the drop, we are talking about 2 ppm calcium per day and 6 ppm magnesium. Neither is really very much, and you could probably boost both higher all at once and just let them ride down.

 

[Triggerjay]

Sounds good. Tank is 65g +-5g in the sump.

 

[Triggerjay]

Currently dosing 6ml alk and 9ml calcium. Mag gets dosed manually monthly as needed

 

[Randy Holmes-Farley Verified Community Expert]

With 6o gallons water volume and replacing 750 mL of salt water a day with fresh will drop 40 ppt to 35 ppt in 41 days.

 

[Triggerjay]

Man I hate math! That's what Randy's are for! Thanks man! So if I double that, it should take about 20.5 days.. Which should be reasonable.

 

[Randy Holmes-Farley Verified Community Expert]

Man I hate math! That's what Randy's are for! Thanks man! So if I double that, it should take about 20.5 days.. Which should be reasonable.

Yep.