I hate to say this but your tank is way overstocked. A 6-gallon can't support much life and isn't good for much besides a single betta, a few guppies, a few minnows, etc. The pleco is messy and will get too large for the tank. I would remove him since he's undoubtedly polluting your water.
The "glofish" are actually genetically altered zebra danios, as you might be aware. Zebra danios are very active fish who appreciate clean, cool, fast-moving water. They're not appropriate for aquariums under 20-gallons, should be kept in large schools, and need a well-filtered, aerated tank.
It sounds like someone gave you poor advice along the way, hence your stocking
. The only way to really save the danios would be to get them into larger living quarters and away from the pleco (what kind of pleco is it, by the way?). In the meantime, do frequent water changes and put the sick guy back in the main tank (he will die if you keep him in the cup). Bring a water sample into your local pet store and have them test it, they should do it for free.
The Eclipse tanks are nice but the fish you have are just too large/messy/active. I would consider re-homing the fish/moving them to a larger tank. Then think about stocking the Eclipse with some small, less active fish such as celestial danios, cherry barbs, white cloud minnows, a single betta or pygmy gourami, harlequin rasboras, pygmy cory cats, etc. Not all of those together but you catch my drift. Possible invertebrates could include freshwater nerite snails, amano shrimp, or cherry shrimp.