The first five minutes
A leak forces fast decisions, and the order matters. Fish are safer being moved early, while their tank still holds most of its water and they are calm, than being chased later in shallow, cloudy water as the level drops. Before you touch anything, look at where the water is going: if it is anywhere near sockets, cables or a power strip, switch the tank's power off at the wall first. Then get your containers beside the tank, part-filled with tank water, so the catching itself is the only task left.
Containers and water: use their own
Move fish into sturdy food-safe tubs, buckets or strong fish bags filled with water taken from their own tank. Their own water matches the temperature and chemistry their bodies are adjusted to, while fresh tap water — cold, chlorinated and different in character — adds shock on top of fright. Fill containers only part-way so they can be carried without sloshing, and put a lid or cover on each one: frightened fish jump, and a covered container also stays warmer and feels darker and safer to the fish inside.
Power off before hands in
Water and electricity are the hidden danger in every leak. Heaters crack, filter casings weep, and a puddle under a cabinet can reach a power strip on the floor long before you notice it. Switching everything off at the wall before your hands go into the water removes the risk entirely, and it costs the fish nothing — they will manage without filtration for far longer than they would manage a shock. Keep a towel under your feet as you work so you are never standing in the spreading puddle in socks.
Catching without panic or injury
Work slowly even though the situation feels urgent. Fast chasing in a draining tank exhausts fish, tears fins on decor and drives shy species deep into rockwork where they cannot be reached. Guide each fish gently toward a corner with the net, or better, coax it into a submerged jug or container — many fish swim into a clear box they cannot see as a trap, and never leave the water at all. Move the rock-hiders last, lifting decor piece by piece, and do a final slow count against the number you know should be there before you declare the tank empty.
Holding them safely until the tank is sorted
Set the containers somewhere level, quiet and away from the wet zone, with lids on and a towel draped over them to keep things dark and calm. If the wait stretches beyond an hour or two, a battery air pump with an airstone in each large container keeps oxygen up; small containers can simply have their lids cracked and the surface rippled now and then. Do not feed — uneaten food fouls a small volume fast. Extra hands speed all of this up, and KinsLe's Circle of Love links you to nearby pet lovers who can be reached when an emergency like this needs people quickly.