Answer: From your description I can't give an exact diagnosis, unfortunately. Although I assume you have watered them regularly and well and have increased the amount you apply to accommodate the larger size of the plants, it is possible they are just wilting from water stress. At eight inches, they are probably outgrowing their peat pots! If they seem root bound, you should consider moving them to a larger size pot.
If water is not the problem, I would suspect insect damage rather than disease or fungus. I say this because they are inside and have been healthy so far. Check very carefully, especially on the undersides of the leaves and where the leaves join the stem.
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