Well, I'd start with a 15%. Then You can do 10% every other day. If they have been surviving for the last 2 weeks then it's not lethal, but they prolly don't like it. My guess is that by week 4 everything should be stable. Provided you don't add anymore LR from that source, lol. You should go get a water sample from there tank and test it. Ummm, that was only a joke.