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Hello. Welcome to Just Answer.
I just looked up the aquarium bulb, as I wasn't familiar with them previously.
However, it appears they have the same ratings as a standard 40 watt T12 bulb.
So, I don't see an issue with having that bulb and a standard bulb in the same fixture, powered by the same ballast.
How old is the ballast?
Do you have a volt meter?