Agreed that they are too wet. Hold off on the water for a while and let them dry a bit or they may dampen off and die. Make sure your solo cups have lots of holes for drainage and I'd suggest watering by hand until they are bigger and growing well. About a shot glass of water is usually enough for a day or two for seedlings IME. In soil, you probably don't need any nutrients for 2 + weeks, then start with about 1/4 the recommended dose of the FF trio, working up to about 1/2. I would not use any of them on their own, use the feed chart for the ratios.
