What Forty Gallons Changes
The jump from 20 to 40 gallons is the jump from accommodation to negotiation room. In a 40-gallon, several angelfish can each hold a loosely defined area, and a fish that loses an argument can retreat far enough for the argument to end. That single fact changes everything about group keeping, because angelfish conflict is rarely about hatred; it is about distance. Give them distance and the lip-locking and chasing that plague smaller tanks settle into brief displays. This is the smallest size where keeping more than a pair stops being a gamble and becomes a method.
The Group Route: Five In, a Pair Out
The classic way to obtain a genuinely bonded pair is to buy four to five juveniles, or six if the tank is a long 40-gallon breeder, and grow them on together. Over the following months the group's hierarchy sorts itself out, and eventually two fish begin behaving differently: swimming together, chasing the others away from one spot, and cleaning a broad leaf, a piece of slate or the filter intake with their mouths. That is a pair forming. Once they have spawned or are clearly guarding a site together, the remaining fish should be rehomed, because the pair's defence of their territory will now be aimed at former tankmates who can no longer be allowed near. Shops and local clubs take healthy young angelfish readily, so plan those homes before you buy the group.
Keeping the Pair With a Community
If breeding is not the aim, a bonded pair in a 40-gallon can still share the tank, provided the community is chosen for the levels the angels do not use. Corydoras and other peaceful bottom dwellers go almost unnoticed. A school of sturdy tetras of a size too large to swallow holds the middle water, though during spawning the pair will push the school to the far end, and the tank is big enough for that push to be survivable rather than lethal. What a spawning pair cannot share with is another angelfish, or any fish that insists on occupying the pair's chosen corner.
Plants and Sight Lines Do the Peacekeeping
In a 40-gallon, aquascaping is a stocking tool. Tall plants, a piece of branching wood and a broad-leafed plant at each end break the tank into visual zones, so a subordinate fish can be out of sight rather than merely further away. Out of sight ends chases that open water lets continue. Leave a clear swimming lane along the front so you can watch the group, since the first sign of a pairing, or of one fish being singled out, is behavioural and easy to miss in a jungle. Check weekly that every fish is feeding; the one hanging back at food time is the one the group's politics are squeezing.
When Four Adults Still Fight
A group of four unrelated adults bought at full size is a different proposition from juveniles grown on together, and it is the one arrangement at this size that regularly fails. Adults arrive with their hierarchy unsettled and their size at maximum, and a 40-gallon may not give the lowest fish enough refuge. If you inherit or buy adults, introduce them together, rearrange the decor at the same moment so no fish owns the layout, and be ready to separate a persistent victim. The tank size is right; it is the group's history that decides the outcome. Put a follow-up question about juveniles, a proven pair or an adult group to KinsLe's Kinny; he answers using the KinsLe Answers library, so the choice is settled before any fish are bought.