Sump levels constantly changeing?

[othicx]

So I have a question for everyone. I have been hired to maintain the tank they have in Joey's only and they have had it running for about a month now. The one thing that we have noticed with it is that the sump level will drop about 6 inches every day but then rise again every now and again. The auto top off has been turned off as it was filling the sump up to much. I was thinking it was condensation as it is pretty warm in the store and also they have a topper on the tank with 4 MH lights in it. What do you all think could be the issue if not condensation?