when I swapped out the gravel in my tank and siliconed in the divider I didn't have another tank to put the fish in for the duration.
I wanted to keep as much of the water as possible so I moved most of the water, filter, fish, heater and plants into a big chilly bin (plastic) and they lived in there for a couple of days while the silicone went off.
They didn't like the white bottom of the bin, it took one of the otos a couple of days to get his full colour back but they were otherwise fine. The chilly bin was deep enough for the filter and heater to keep things as normal as possible during the transition.