Need some help! I'd bought RO in from my lfs, but having had a tds pen arrive today I'm measuring it in the containers between 230-280!! I've put quite a few shrimp in now so want to sort this before they die. Only option I can see is to buy myself a filter. If I change 50% a week then I need 40 litres, so can get away with a small unit. The question is what do I need to buy? Seems I'm looking at £100-150?