「だから得だかどうだか早く水槽を見なさいって言ってるでしょっ。」
Therefore something about profit/advantage I told you to look in the tank soon, right?
I can't understand 得だかどうだか nor how it attaches to the rest of the sentence.
It looks a bit like the かどうか that means 'whether or not', so may be "whether or not you make profit I told you to go look in the tank", but I thought the pattern was Xかどうか or Xだったかどうか for the past, but definitely not Xだかどうか. And then there's the other だ. I'm rambling and I'm stuck.
In meaning, one could say:
「得｛とく｝だかどうだか」＝「得なのかどうなのか」＝「得なのかどうか」＝「得かどうか」
= "(to find out) whether or not it is profitable/beneficial/economical, etc."
Without more context, there is no way I could translate the 「得」 part. 「得だかどうだか」 functions adverbially and it modifies 「見なさい」. If it helps any,「得だかどうだか」 literally means "Is it 得? How is it?".
「～～だかどうだか」 just sounds more informal/colloquial than the others I listed. Not everyone might use it, but absolutely no native speakers will be surprised if anyone said it in informal speech, either.
In fact, you will find a very similar expression in this well-known children's song.
