Safe in what way? If you mean chemically in the water then yes, it's fine. I've used it in my tanks for years on fittings. Just let it cure for a day or so first.
If you mean leak-wise then it's only as good as the way you apply it! If you're worried then you can use a sealant such as Gold Label pond sealant which is completely safe and will even cure under water. It won't set solid like araldite though.
I was going to pick them up from the Newcastle store next time I was in the vacinity. But this morning my next door neighbour, who was around yesterday while I was putting working on the reactor, brought round one of these he had in in his shed:
I'm using 40mm solvent-weld fittings that I bought from Screwfix. Instead of introducing the CO2 from the top, I think I'll introduce it from the side with another T-peice in the middle, and will have a bleed valve in the top instead of the CO2 inlet, like this:
The reactor will also act as an in-line heater too.
I'm waiting for delivery of some 40mm clear acrylic pipe.