late to the table here , but do you have good air circulation, Add a fan, it will help cut down on overly wet leaves,
it does look a bit damp in the grow area, are these in a tank or in the open air , crowding can be a real problem can you move the plants a few more inches apart, or to another grow space.
i grow in a terrarium almost exclusively, and have to watch out . for the same kind of problem, IMO its the damp conditions , especially if you see no bugs, but dont rule them ou, t i would still do what tony has suggested. it cant hurt.
If you do use a fan be careful not to dry them out of they are used to a high humidity environment,sometimes a a fan with a bright light can kill them quick if your not watching.