The Pi can run all sorts of things at the same time, limited only by the resources available to it. It (a Pi2) should easily run the streaming and aquarium monitoring programs at the same time.
Just be aware that if the aquarium monitoring is important, and the streaming program uses a lot of CPU or storage space, you might have the situation where the monitoring program is interrupted or stalled for a brief time. Consider running the aquarium monitoring on a separate Pi (e.g. PiZero ?). Get them both working first and see how it goes.
Good luck (and keep reading

)