One system, not five separate tanks
A multi-tank home is really one system connected by its keeper. Pathogens rarely jump between tanks on their own; they travel on the net you used an hour ago, the siphon that never dried, your wet hands, and the new fish that looked fine in the shop. Prevention therefore means controlling that traffic deliberately, in three places: what comes in, what moves around, and the order you work in.
Design it once, write it down, and the system protects every tank at once instead of relying on you remembering which net touched which water.
Quarantine: the gate everything passes through
Every new arrival, fish or plant, spends its first weeks in a separate quarantine tank with its own water and equipment, no exceptions for a bargain or a gift. That window is where most imported problems declare themselves, in a small bare tank where you can watch the fish eat, inspect its fins and body at close range, and treat if needed without medicating a whole display community.
Only a fish that has fed well, stayed clear-skinned and behaved normally through the full period graduates to a display tank. Keep the quarantine tank running permanently rather than setting it up in a panic; a cycled, ready gate is what makes the rule easy to keep.
Tools, hands and the working order
Give each tank its own net, or accept a discipline: shared tools are disinfected and then dried completely between tanks, because thorough drying alone defeats many aquatic pathogens. Algae scrubbers, siphons and buckets follow the same rule, and your hands get washed and dried between a suspect tank and a clean one.
Then fix your rounds. Always service the healthy, established tanks first and any tank that is new, quarantined or under suspicion last, so your route through the room never carries trouble from the risky water to the safe water. If one tank does fall ill, it gets its own dedicated set of everything until it is clear.
Buying like it matters, because it does
Choose fish from shop tanks that are clean, unhurried and free of dead or shimmying occupants, and watch your specific fish feed before you commit where the shop allows. Avoid the temptation to rescue a sorry-looking fish from a rough tank; sympathy imports are the commonest way disease enters a good system. Buy in small batches that your quarantine can hold comfortably, and time arrivals so you can actually observe them daily.
Ask where new stock came from and how long it has been in the shop; fish that have rested and fed there for a while are a safer bet than same-day arrivals still stressed from shipping.
Turning all of this into standing Care Teams duties in KinsLe, with the quarantine checks and tool-cleaning steps written into the tasks themselves, keeps the system intact on the days a family member or sitter does the rounds instead of you.