You shouldn't be feeding them until they are rooted, in my opinion. I soak the rockwool real good in a Grozyme mix, leave them alone for like at least 7 days. Only wiping off the dome and misting them daily during this time. I have had some clones take up to 20 days to root, but my average is 7 to 10 days. I even had some root in 3 days. Once they are rooted and in another medium ie: larger rockwool or promix I start to feed them.
As for that heat mat, I don't use it anymore, way too many issues with those to mention. Just maintain the room temps at 26c. Works perfect for me all the time, I get about 99% rooted.