The temp fluctuation for the fish is a issue likely to stress them then cause disease, without a storage container your in a hard spot , I always add salt to my water changes the fish love it and haven't noticed any bad affects on the plants in 3 tanks , are your plants easy grow would they be ok with the softened water so you could add the correct water temp for changes.
You could install a under the sink on demand water heater but that involves some expense, the best option is to find a safe way to accommodate the water storage
You could install a under the sink on demand water heater but that involves some expense, the best option is to find a safe way to accommodate the water storage
