Label at the moment of opening
Expiry trouble rarely comes from old stock bought carelessly; it comes from a tub opened months ago that nobody can date. Build one habit: the marker lives with the fish food, and nothing is opened without its lid gaining two dates, the day it was opened and the use-by date printed on the packaging. Frozen foods get the same treatment on the outer bag, since individual blister packs carry no dates of their own once separated. This takes seconds at opening and removes every later argument about how long something has been on the go.
Decanted food is where labels matter most
Many keepers decant flakes, pellets or granules into small pots by the tank, and those anonymous jars are the true risk in the cupboard. Label every decanted pot with what it is, which tank it belongs to, and the use-by date of the batch it came from, not the day you poured it. Different foods look deceptively alike once loose, and a herbivore pellet fed to carnivores for a month, or a stale batch topped up with fresh so the dates blur, both start with an unlabelled jar. Never mix old and new stock in the same container; finish or discard the old first, then refill and relabel.
Keep foods and treatments apart, oldest forward
Store dry and frozen foods in one box and water treatments, conditioners and test reagents in another, because a bottle grabbed in a hurry should never be the wrong kind of bottle. Inside each box, run a strict front-to-back order: oldest stock at the front, new purchases behind, so the natural reach takes the right tub. Test reagents deserve special honesty, since an expired kit produces confident, wrong numbers that send you chasing problems the tank does not have. Note the opening dates of liquid test bottles too, as many drift in accuracy long before they run out.
Build a discard rhythm, not a discard crisis
Check the whole store briefly whenever you buy new food, and throw out anything past its date there and then rather than returning it to the shelf on a promise. Expired food loses nutritional value and can foul water as it breaks down, and out-of-date treatments cannot be trusted to do the job their label claims at the moment you need them most. When you use the last of a current batch, write the replacement on the shopping list the same day, because the gap between running out and reordering is when mystery jars of forgotten food get pressed back into service.
Make the dates visible to everyone who feeds
The system only protects the fish if the next person can read it, so keep the dates on lids where they face upward, rewrite any label that has smeared, and keep the marker in the food box rather than the kitchen drawer. Listing each product and its use-by date in the tank's KinsLe Pet Passport as well means a holiday sitter, or you at the shop deciding whether to restock, can check what is current without opening a single tub.