「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.