Three weeks is a human problem, not a gadget problem
Long trips fail tanks through slow drift: evaporation lowering the water toward the filter intake, a feeder drum emptying unnoticed, algae creeping over the glass until the fish are hard to count. No single device watches for all of that, so the backbone of a three-week plan is a sitter who physically stands at the tank at least every two or three days. A neighbour, a professional pet sitter or a fellow fish-keeper from a local club can all work; what matters is that they see the tank often enough that a small problem is still small when it is found.
Maintenance belongs several days before departure
Do every disruptive job with time to spare. A partial water change, the filter media rinsed gently in removed tank water, algae scraped and hoses checked, all finished three or four days before you leave, gives the aquarium time to prove it is stable and gives you mornings to notice a cloudy tank or a rattling impeller while you can still fix it. The night before the trip, the only jobs are topping up the water level, confirming the light timer's cycle, and one last normal feed. A tank that has run untouched and clear for three days is the tank you want to leave for three weeks.
Feeding: portions decided by you, not the sitter
Remove judgement from feeding entirely. Measure each feed into a small labelled pot, one per scheduled feeding day, and hide the main food tub so topping up from it is impossible. Pair the pots with a tested automatic feeder only if the stock genuinely needs food on the days between visits; otherwise the sitter's pots alone are simpler and safer. Write down, in words the sitter cannot misread, that uneaten food after a few minutes means the next portion shrinks, and that a missed feed is never doubled later. Overfeeding across three unattended weeks fouls more holiday tanks than hunger ever has.
Spares, failures and the emergency ladder
Assume one thing will break and decide now what happens then. Leave the spare heater, spare batteries for the feeder, a net, a bucket and water conditioner where the sitter can find them, each labelled. Write the emergency ladder in order: what the sitter can fix themselves, swapping in the spare heater; what means a phone call to you, cloudy water or a fish loss; and what means calling the aquatics shop whose number you leave on the sheet, a failed filter with you unreachable. A sitter who knows the ladder acts early instead of hoping a problem resolves itself.
One handover the sitter keeps for the whole trip
Twenty-one days is too long for instructions that live in a text thread, so consolidate the routine before you go. A KinsLe Care Pass hands your sitter the feeding pots schedule, the every-few-days check, the spares list and the emergency ladder in one place attached to the tank's details, and because it sits with the pet's profile, you can adjust a portion or add a note from the other side of the world if the sitter reports something changing.