I'm sorry about the slugs!!! How irritating. The beer works. The containers that are sold for the garden usually have a cover so that sprinklers or rain does not dilute the beer. There usually are holes or space for the slugs or snails to climb into and drown. You might have to make a smaller version for the tank. But this should be easy for you Marcin, with all that you are capable of.
May I ask. What type of orchids did you get? Do you have pictures, how long have you had them in your tank, and how are they doing beside the slug issue?
Thanks!