yea weld-on (or aceton, although i'ld prefer the weld-on) and then some nice sealant should work.
A shame you don't have any pictures of your block, care to describe it? Always interested in waterblocks
For the water you would be better off adding something to kill the algae, antifreeze, bleech, ... I don't think any of these eat plexi, you could verify it with a spare pice of plexi though, if you want to be safe