How large is this tank? Trout usually inhabit rapidly-flowing streams and rivers, and also ponds (though mainly, they are stocked there). I would think it would require a huge aquarium to accommodate trout -- at least 100 gallons. For that many, more like 300 gallons. And good filtration and current. Riverbed-like substrate.
Do a water test and find out the chemistry. If those trout had been there three years, something has changed, or else they're beginning to finally succumb to stress.
20 gal. high: planted; 8 white cloud minnows, 10 RCS, 2 blue shrimp, several snails; AC50, Azoo air. 65 gal: planted; 7 rosy barbs, 6 glofish,, 2 zebra danios, 6 rosy red (fathead) minnows, 3 dojo loaches, several snails; AC110 x 2.