From when they were fuller to now (looking stringy), did you change/modify their feeding regiment?
When they are searching for food they tend to stretch out, or If they don't have proper lights/aeration.
For instance, with my 2. When they get thinned out that's one of the first signs they show that lets me know it's time to replace my T5 bulbs. Once I do that, within a couple of hours they're plumped back up.
Frank