I dont use the water during the summer period due to accessability
St Ives is really tiny and with the sheer number of people its practically impossible to get the water. Saying this i havent changed the water in my 100 gallon since may.. i am changing it tomorrow actually.
As for treatments. I collect it and then bring the temperature up by adding some tank water and placing heaters in the buckets. I really need to get the tank up to temperature withing 5 or 6 of hours or the life in the water starts to decay. (Natural water doesnt keep as well as artificial water).
I do a 10 gallon water change normally and if i can get the sea water within 5 degrees of the maintank water then when its added the tank temperature does drop at all (not by my readings anyway)
I dont test for anything other than Nitrates and phosphates. I have yet to have any other reading other than zero from this location. If you are familiar with Cornwall then a few miles down the road is aplace called Porthleven. far easier to collect water from but when i tested this water the nitrates were over 100ppm and phosphate were off the scale
The reason for this i feel is because of the oil tanker that crashed just off of hear some years ago (The Amoco Cadiz). The oil it dumped is still just a few feet below the surface of the sand. The water looks lovely and clean but it most definately isnt
As for the location you might be travelling too. I know a person who lives in TOquay and he use Natural sea water (he is a marine biologist) the water there is very good also i am told.