My understanding of the term epiphytes is plants that grow attached to rock or driftwood. Is that correct? The most common types of these plants are anubias and java fern.
Aquarium plants are cultivated emersed, ie partially out of the water. This means they have easy access to atmospheric CO2
, can grow quicker, and makes it a more commercially viable proposition. You take those plants, put them completely underwater, cut off that atmospheric CO2
and the plant needs to adjust to its new environment. Its leaf structure needs to change to one that can better take up CO2
from the water. It does this by using up the stored carbon in the leafs to stay alive, hence the melt, while growing new leafs that are more suited to its environment. Its possible you may lose all your original growth, but its new growth you should look for. 3 weeks isnt really enough time to judge if your plants are going to be OK. Remove dead and dying growth so the plant can concentrate its efforts on new growth.