It is normal to experience moulding, in such situation. You can try to spray it with a solution of salicylic acid (you can use aspirin).
I remember I saw somewhere a similar isue that has been solved this way. Moulds are senzitive to soft acids, mostly to volatile acids
like acetic, propionic and formic acids. The best would be propionic acid but I cannot say from where you can get it. Anyway, take
care of the plants if you choose to use acids. There are also anti-mould products, that contain such active substances, in buffered form.
Maybe if you give a search, you can find some. I remember I used some time ago a product from Bison. Maybe this can help.
Thanks for your reply. The question now is it worth to risk acid on the plants to solve the mold issue. Do you know if the mold can do any harm as long it doesn't touch plants? Will it dissapear once the tank is flooded?
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