Background:
I mixed a bag of lettuce nutrient (powder form) into 5 gallons of water, as my concentrate--as per the directions. I mixed it in a 5 gallon bucket and covered it with the attached lid, tightly.

When I came back to mix another batch of solution 10 days later, there was a thick layer of green powdery mold floating onto of the solution. I skimmed it off and put it, uncovered, in the sun.

1. Do I have to dump the solution?
2. What can I do to prevent it in the future?
3. Should the plants that have already been fed with this solution be destroyed?

Any help or information that you can provide will be GREATLY appreciated.

Best,
Effie

Always clean your bucket with bleach before filling it with the stock solution.

Put your solution always away from sunlight. Mold grows with sunlight, in dark conditions it can't grow.

It's better to prepare 1 - 2 liter of concentrated solution. Five gallons is too much, unless you have a lot of plants.

If you can, use distilled water to prepare the concentrated solution.

You don't need to destroy your plants, try washing the entire system.

And check the pH of your concentrated solution, it should be acid.

Second, that is a mold it will be extremely hard to get rid of because just one spore starts it all over again. Also mold does not need light to grow, I'm sure jamesvlad was thinking of algae winch does need light. It's always recommended to keep your nutrient concentrate (even solution) in a cool dark location for that very reason.

(1) If it is just a residue left from mixing, then it wont mater. Though it would indicate your nutrients didn't mix correctly.

(2) If it is alage, then just skimming it off, and keeping it in the dark is all you'll need to do.

(3) If it is a mold, you'll likely never be able to get rid of it from your concentrate. I cant really think of a safe way to even try unless you tried mixing in about 1-2 cups of 3% hydrogen peroxide (H2O2) per gallon. I use it a lot in the nutrient solution itself, but have never used it in a nutrient concentrate. I just don't have any idea if it will affect the mineral elements in the nutrients being so concentrated. If it is a mold, and in your concentrate, you'll just keep spreading it to your plants every time you use it.

If you aren't sure, I would recommend to take some pictures of it, then e-mail them to the nutrient manufacture to see what they think. Even if they cant tell you for sure what it is or how to fix it, they should at least be able to tell you if it's normal or not.