I managed to grow a few Drosera rotundifolia plants from seed.
They've been doing nicely outside under sheltered sunlight since their sowing (I'm in temperate zone so climate fits their needs), never kept them inside a dome (so that I wouldn't have to worry about mold) but always used an open tray filled with 1/4 inch (6mm) of water (TDS 18 ppm). The soil is pure blonde peat fit inside an icetray (with holes drilled in the bottom).
I just fed them with insects and watched them grow. My largest ones (shown in this picture) are about 2 inches (5 cm) in diameter leaftip-to-leaftip.
Fungus gnats did often fly above them and I watched sometimes small worms swimming (maybe their larvae) in the shallow water but they never seem to have bothered them in months.
Now all of a sudden the once oblique leaves flattened on peat and the center of their rosette somewhat turned brown.
What could it be? Could you please tell me how to save them?