Re: what to top it up with?
#7
I'd get some RO water and keep it topped up, if they're already in a smaller container then oxygen levels will be stressed and as the water evaporates the water salinity increases and that again reduces the oxygen carrying capacity of the water as well as introducing possible osmotic stress (and dehydration) on the fish.
If you're not checking salinity then at least keep the water topped up.
I assume this is a temporary home for them while your friend is on holiday?
Are there any invertebrates or corals in the tank?