The nice thing about Blynk is the ease with which you can construct a phone app. I believe you can set up your own Blynk instance on a local server too.
Personally, I wouldn't want to give that piece up and have to write a native phone app. On the other hand, you can run a web server on the Pi and control your aquariums from a browser on the phone. You would need some kind of login to identify the individual users so that you know which aquarium they are trying to control.
You can run MQTT on the Pi too, and have each Aquarium get its instructions from there. I'd be more inclined to stand up a web API though so that the Aquarium controller can ask for status when it wants it, rather than having to rely on a persistent MQTT connection.