When ever i have a problem like that on a canopy, or if the canopy starts to get fogged up, I use a razor blade and a little water. If you use water and use it gentily, it wont scratch the glass. It might take a few hours, *frown but you probably will get a lot of it off.
If it is calcium deposits, try vinegar and it will help to remove them with work and I mean work and it will not make your aquarium unusable. It will also disinfect the aquarium to use vinegar in it and then wash, wash, wash with clear water. It will take a while to take the deposits out. There is a product to remove calcium from an aquarium that is aquarium safe but it is also a bit expensive and does not do a better job than the vinegar so that is why I do not recommend it.
i might as well weigh in also vinegar works great on hard water deposits, which this appears to be, i buy the cheapest vinegar i can find, because its less refined, and therefore contains more of the acid required to remove lime. I just pour a little full strength on a plastic scrubby pad, and go to town, then rinse well with clear water...Bingo
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