Your best using whats called "de-ionised" water, its intended for use in car batterys, so you can get it at places such as halfords. Its effectively boiled and filtered water to remove all the impurities which also makes it ph 7 (or neutral) so it wont affect seals. It does cost but....... it wont kill your seals !!!!!!! The other options to ask the science department at your school nicely for some distilled water, almost the same thing (or so my teacher says ) hope that helps joshywa