What you gain by selecting a task for automation varies depending on the work you had to do to do the task, how often you do the task, and how much effort you end up putting into the automation and testing to ensure the automation is working and what is it going to cost you to do that automation.
In your aquarium world many tasks are repeated over and over. Any of these tasks is a candidate for potential automation.
Some things that are commonly performed are things like feeding various live, frozen, pellet or flake foods, water changes, pH monitoring and adjustment, kH monitoring or adjustment, fertilizer dosing, temperature adjustments and more. Some types of automation are simply just methods of efficiency and distribution to improve a manual and sometimes complicated process. In these cases the automation is not really to replace the human work but to make it more efficient or accessible.
Sit back and make a list of everything you do. Think about how it would be better if you could have a way to do it without you touching it. Just because you have one type of automated way of doing something don't just accept that as the final answer. There may be better, faster, more efficient ways of doing it.