Write numbers, not impressions
A carer cannot copy quite warm or fairly bright, so every setting in your document needs to be a value they can check. Record the heater's set temperature and, just as important, the reading your tank thermometer actually shows at that setting, since the two rarely match perfectly and the thermometer is what the carer will see. Note the light's timer hours and its brightness level if it dims, the filter's flow setting, and whether the air pump runs day and night. Where a control is a dial without numbers, mark its correct position with a dot of tape or a pen line and write that down: the mark is the setting, and the carer restores the mark, not a memory.
Photograph everything as it should look
Words fail on plugs, valves, and pipework, so back them with photographs taken in good light from the carer's viewpoint. Photograph the heater dial, the light unit's display, the filter's flow lever, the tangle of plugs labelled with which socket feeds what, and the water level the tank should sit at. A carer faced with a silent filter at midnight can compare your photograph to the equipment in front of them and spot the one plug that has worked loose. Keep the photos with the written settings, in the same order, so the document reads like a tour of the cabinet from left to right.
Document the routine, not just the equipment
Settings hold the tank steady only if the daily routine around them is copied too, and feeding is where carers do the most damage with the best intentions. Measure each day's food into labelled pots before you go, and write down that one pot is the whole ration however hungrily the fish beg, because a fish-sitter's classic disaster is a week of double feeds fouling the water. Record which days get which food, how much water to top up and with what treated water, and the short list of things never to do: no adjusting the heater, no rinsing filter media under the tap, no extra light hours for a better view.
Include the 'something looks wrong' page
An exact copy of your settings cannot cover a fish death, a leak, or a power cut, so add one page of what good looks like and what to do when it changes. Describe the tank's normal sounds, its normal smell (barely any), and the fishes' normal behaviour at feeding time, then the three or four actions that cover most emergencies: who to call, where the spare heater and air pump are, and permission to do a partial water change with the treated water you have left ready. A carer who knows what normal is will spot abnormal early, which is worth more than any setting on the sheet.
Hand it over as one complete packet
The document only works if the carer actually has it, complete and current, when you are not there to be asked. Sending the settings, photographs, routine, and emergency page together through a KinsLe Care Pass gives your sitter temporary, clear access to the whole packet for exactly the days you are away.