i have seen folks use pre-formed ponds as sumps?

[goodreeef]

how do you control the salinity fluctuations due to a large surface area. i.e.

-If you have a 1/2" water fluctuation in a 10 gallon footprint that maybe a one gallon of evaporation

-but in a say 90 gallon footprint that would be 2.5 gallons evaporation. 

in a preformed pond that could be 10 or more gallons.

 

I was thinking about using this as my sump for my 300 gallon display.

 

http://www.thepondoutlet.com/home/tpo/page_6281_119/pre-formed_pond_by_laguna.html

 

does anyone see an issue with using a tunze top-off with this. In my expierence the tunze light will change to red when it has pumped too much?

 

will this result in too much salinity fluctuations?

[ctenophore]

The salinity fluctuation still won't be significant enough to bother anything. Just do the math; the percentage change is probably much less than your refractometer can measure. I used to use a tunze a long time ago but it took too long to fill and would time out. I use two 4'x4' polyethylene totes linked together as a sump. That's 32sqft of variable surface area. It takes my ato pump (peristaltic) about 3 hours to fill it 1/2". The salinity change is not measurable with my refractometer.