Sounds like you have the beginnings of pythium (root rot). Ice blocks will work but it will dilute your solution as they melt, which could be good or bad. I've been using water frozen in 2 liter bottles to keep my temps down. Its sort of a hassle, and you have to swap them out every 4 hours or so and since they take like 12 hours to freeze, you'll probably need to have a rotation of about 3 or 4 of them. But your only other option is to get a Coolworks Ice Probe because the max temp your res can be is 75 or the roots will rot and your plants will continue to suffer.