extremley slow growth. distilled water???

Hi i recentle asked a question about my slow growth and yellowing of seedlings.I had some good responses and have come to the conclusion my seedlings need distilled water untill they are more mature.I use water butts and i have a fish tank heater in there at the moment as they are out side .every time the butt is half empty the heater is completley white with scale.I have been through all my other possibilites and cant seem to find an answer.i have posted the pics again to see if any one would agree it sounds like they need distilled water. I had some clones that went yellow and would not grow recently.
when they became a foot tall they started to come round with a little help from some nitrogen spray. I HAVE GOOD VENTILATION and 1000W Mh and always use bio bizz or canna terra seed mix.. so iam hoping distiledd water must be the answer.????????????? massive headache

What do you mean by slow growth? how long have the not grown? from the pics I saw in the other post, if there a light shade of green that's normal. As for the water leaving scales on the heater that's normal as well water has salts in it in the form of minerals and vitamins.
If you think you have hard water you can use a 1/4 tsp of Epsom salt per gallon of water. This will soften the water and add a little magnesium sulfate, and see what changes if any occur. This will not hurt your plants.

Keep us posted.