Sounds like your solution is contaminated with bacteria or algae.
Try changing the nutrient solution. You could also add a small amount of diluted hydrogen peroxide to the solution.It should kill bacteria and algae plus it provides oxygen to the solution.

Thanks for your reply Habman

Actually, this happened on the first day, with freshly made nutrient. I used Miracle Gro in the strength indicated on the packaging.

It's not likely, therefore, that the problem is bacteria.

Stop using miracle grow in Hydro, it is designed for dirt.

The main difference between hydro and soil fertilizers is the nitrogen source. Ideally, only calcium and potassium nitrate are used in hydro, where soil fertilizer often uses urea and ammonium based nitrogen. The urea requires a soil orgainsm that does not exist in a hydro system to convert to a usable product. The ammonium nitrogen can cause root damage if too much is used. It seems to work OK with up to 15% of the nitrogen source. Ammonia nitrogen can be used in moderate amounts to bring a high PH down.

Thanks for the info Willard3