Top up water

As the growbeds are not fully planted, and the area is really a glasshouse, a fair bit of water evaporates each day. The growbeds get quite hot to touch in the middle of the day which clearly accentuates the problem. .
To keep the system running well and to ensure the plants get adequate water top up water is needed fairly often- really about every couple of days. The plan is to fill the growbeds to within a couple of centimetres of the top of the gravel each flood cycle.
I have a 20000 litre rainwater tank collecting the water from the shed and I have run a pipe from that tank to just over the fish tank.
Up until the addition of the top up barrel I simply opened the big white gate valve and ran in water for a short while. The problem with that arrangement was that I didn't know how much water I was adding and more importantly on one occasion I forgot the water was running for about 10 minutes and ended up adding far too much. I didn't overflow the fish tank but I did have to reset the float switch which was a pain.
So I have added a 200 litre blue barrel which has a sight tube which enables me to add, and record, an accurate volume of water when needed. At this stage it looks like I need 50 litres every second day. I doubt that I will need to add anything like this much in the cooler months but it is still likely that the extra water needed over a year could be as much as 7000 to 9000 litres. We have had a fairly wet last couple of months and the 20000 litre tank is still full so that much water will not be problem. Posts to several of the online aquaponics groups suggest that an aquaponics setup uses only about 10% of the water of a dirt garden. I have no idea if that is true but I do know that we had real trouble with enough water for our dirt garden the last couple of years wheras we look like having no trouble with water with this new venture.