When you set up an aquarium, be this the display aquarium, sump, [tag-tec]refugium[/tag-tec] or any other type of aquarium one thing you must do prior to mixing the expensive salt with the again expensive reverse osmosis water is to test that the aquarium does not leak.
This is exceptionally simple to do. All you do is fill the relevant aquarium with tap water and leave it for a while.
I know that this may sound like a waste of water but doing so allows you to test the seals in the aquarium. How would you feel if you filled the aquarium with reverse osmosis water, mixed in the salt only to find out that it leaked and that you would need to remove all of this water, store it somehow or even worse have to throw it away.
Filling it with tap water allows you to test the seals. There is no guarantee that the aquarium will not leak. Whilst the manufacturer will make every effort to ensure that it does not they cannot guarantee it. Or if they do provide a guarantee it will be that they either repair or replace it. Either way you have lost valuable water and salt.
Of course filling the water with tap water allows you to test more than just the aquarium seals. You can test your equipment at the same time. You can test the water pumps and the heaters to ensure that they are all in working order. If they are not then they can be returned for replacement.
If you have drilled the aquarium or have had an [tag-self]aquarium overflow[/tag-self] fitted then this initial test fill will allow you to test the plumbing. It wil become apparant quite quickly if you have a leak. You can then empty the aquarium, perform the required repairs and/or modifications and test it again.
Imagine the scenario if you will. You have an aquarium, you have filled it with water, heated it up, added the salt and added the sand for the sand bed. But you did not know that you had a slight leak from the standpipe overflow hole for example in the bottom of the aquarium. In this scenario you would have to empty the aquarium, remove (or move) the sand, perform the repair and then put it all back in again.
What a waste of time, energy and resources.
If only you had tested it.
Testing the aquarium and any relevant aquarium equipment in my opinion is important as you can be safe in the knowledge that the aquarium is water tight and that at this point in time you have no leaks. You may get one in the future as this cannot be guaranteed not to happen but you are off on the right foot.
What to do with the tap water?
Keep it when you empty the aquarium and use it to water the garden – thats what I always do anyway.