They can go a long time without eating. I bought a really big one once 18" didn't eat for over 2 months then slowly started taking pellets.
Going on for 2 months without food did the I.Ds get sick at all, or any harm done that was permanent?
They need to be moved out of that tank ASAP;
Stress from over-stocking is a very good reason why they wouldn't be eating. You'll need a pond (literally) to house them, and their current size is not a reflection of when you should be upgrading. Please rehome them or find a suitable tank for your fish, as they all become very large..
Going on for 2 months without food did the I.Ds get sick at all, or any harm done that was permanent?