Start with who lives in the tank
The new owner needs the residents first: every species, how many of each, roughly how old they are, and how long each has been in the tank. Note the order fish were added where you remember it, because stocking order explains the pecking order a buyer will watch unfold in the first week. Flag individuals that matter: the shy one that feeds last, the pair that has claimed a corner, the elderly fish that should not be chased with a net. A buyer who knows the cast settles the tank faster and misreads far fewer early behaviours as problems.
The water story, in numbers
Write down the targets the tank has been run to, its temperature, its usual pH, and the results of the last several tests in order, so the new keeper can match their change water to your water from the first bucket. Add how the water has been prepared: treated tap water, a mix with filtered water, anything added at each change. Note the rhythm too, the size and frequency of water changes the tank is used to, because a sudden move to a very different routine is a stress the fish did not need on top of a journey. If the tank travels full or part-filled, say what you did and what the fish are accustomed to on arrival.
Equipment, with ages and honest condition
List what is included and how old it is: filter and when its media was last changed, heater and whether it has ever misbehaved, lights and their daily hours, substrate type and depth. Be straight about wear, the impeller that rattles for a minute at startup, the seal you watch, the spare parts in the cupboard that are going with the sale. Honesty here is not just fairness; a new keeper who knows the heater is elderly watches it, while one who was told nothing blames the fish when it fails. Include manuals or links to them where you still have them.
Health history, told plainly
Say what the tank has been through: any illness outbreaks, what signs appeared, what was done, and how long the tank has been clear since. Note losses as well, and when they happened, because a buyer deserves to judge the risk of taking on fish from a tank with a recent problem. This is general history, not a diagnosis of anything current, and anything you are unsure about should be described as observed, not named. Tanks with a clean, well-kept record are worth more, and a written history is the only proof that yours is one of them.
Routines and the transfer itself
Finish with daily life: what is fed, how much, how often, and the small rituals the fish expect, like the light going on before food arrives. Agree what moves with the tank and what stays, and whether you will be contactable for questions during the first fortnight. Because the whole packet, stock, water, equipment, health and routine, already lives together in the tank's KinsLe profile, handing over through Transferrable Profiles means the new owner receives the tank's real history in full, not a summary reconstructed from memory on moving day.