Start by listing the real jobs
Fair shares fail when the list lives in one person's head. Write down everything the tank actually needs, in plain terms: daily feeding in measured amounts, the weekly partial water change, glass cleaning inside and out, rinsing filter media in removed tank water, wiping the lid and light, testing the water, trimming plants, and checking the heater and filter are running. Beside each job, write how it is done properly - the portion size, the share of water changed, where the test kit lives - because a chore done three different ways by three people is not really shared. This written list does a second job too: it shows everyone, often for the first time, how much invisible work a healthy tank takes.
Match jobs to people, then rotate the worst ones
Assign by ability and appetite first. Water changes involve carrying buckets and siphons, so they suit whoever can lift safely. Testing suits the patient and methodical. Feeding suits almost anyone, including children, provided the portion is pre-measured - the classic shared-tank disaster is two people each feeding a full meal because neither knew the other had done it, and the water pays for it within days. Then deal honestly with the jobs nobody wants: nobody dreams of rinsing filter media or scrubbing algae. Put those on a visible rotation, weekly or monthly, so the burden moves around instead of settling permanently on the person who cares most. A rotation everyone agreed to feels fair in a way that ad-hoc asking never does.
Make completion visible
Shared chores collapse in the gap between "I thought you did it" and "I was about to." The fix is a record everyone can see: when a job is done, it is marked done, with the time, where the whole household can check before assuming. Visibility matters most for feeding, where a double meal is worse than a late one, and for water tests, where the result - not just the act - needs to be written down so trends are visible across weeks. Keep the test log with the chore list, and glance at it together occasionally; a slow drift in readings is far easier to spot on a shared record than in one person's memory.
Review the split as life changes
A rota that was fair in a quiet month stops being fair when someone's shifts change, a school term starts, or a second tank appears. Put a short review on the calendar: what is working, which jobs are being quietly skipped, and whether anyone's load has crept up. New tankmates change the work too - fry, a growing pleco or a new feeding schedule all add tasks that need an owner. Treat the rota as a living arrangement rather than a treaty and it will survive. Running the whole split through KinsLe Care Teams, with each aquarium duty assigned to a named person and marked complete with a timestamp when done, gives the household that shared, checkable record without a paper chart on the fridge.