Drain the sump and water out of the display until it’s an inch under the overflow baffles. Point a powerhead towards the surface to increase gas exchange and put your heater into the display. Get yourself a can of “dust off”, weld-on #4 and an applicator. If you have to order it, having the tank like this for a few days won’t hurt your critters so don’t worry. You can manually dose alk/cal/mg if needed. Blast the water residue out of the seams with the dust off, wait 10 minutes for any residuals in the dust off to evaporate, apply the weldon on every seam in that was under water, give it two hours to cure, refill and you’ll be good to go.