Because theres a slim chance that you do rehome them and if they are rehomed it's far too late. Just buy a bigger tank or house the animals that are appropriate.
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If you are considering doing this then you need to understand that these types of fish (I'm guessing tangs, triggers and angels are the most likely candidates) eat, poo and grow a lot, so you need to understand the strain they put on smaller tanks. Juviniles fish can look cute but they often have very fast metabolisms and need more feedings, albeit smaller quantities, than adults.
There is also the point at which they need to be moved on before you affect their health, ie stunting. So my advice is that when you decide they need an upgrade you should have probably already moved them on or upgraded.
Is there a good metric for when a fish's grow will start being stunted by its tank?
It's a difficult subject really and I'm not sure if anyone has really looked into stunting and the longterm health effects. It also seem quite random as I've had fish (admittedly in freshwater tanks) that appeared to stay much smaller than others of the same species.
In truth I'm not sure if stunting is a major problem unless we go to extreme cramped conditions and unless your tank is really tall (like a cylinder) then I think a tang could live quite happily in it, potentially forever (assuming it's a smaller species).
One thing I do know was there was an experiment that measured cortisol in tangs, the stress hormone, and it concluded that after a short period of time that the tangs weren't stressed by having a smaller tank (compared to those in much larger tanks).
So to answer your question, I'm not really sure you can tell for certain, it's just a gut call based on the behaviour of the fish and how quickly it seems to grow. Some tangs can grow really quickly whereas others seem to take forever. So if it approaches it's maximum or sensible size for your tank quickly, it's probably best to move it on earlier. With behaviour we are looking for a contented fish. If it's constantly darting about, doing laps or swimming up and down one end, then it's probably not happy and might do better with more space.
With a wantanabei it's tricky. Every time I've seen them they have been a decent size already which would probably rule it out. I have kept bellus angels in similar sized tanks but they have been small juviniles and I find them painfully slow growers. Hopefully someone has wantanabei experience (there is a sps master on the uk forums called trout who had multiple genicanthus in his tank that I think was a 3x2x2ft, not sure if he's on here though) but they are active planktavores which makes things a little harder.
Boy oh boy this just makes me want to start a larger conversation surrounding husbandry and the arbitrary assignment of minimum tank sizes for fish without clear and defined rationale. Don’t get me wrong, I most certainly do as many of you and try to make conscious determination of what I’m buying, before I buy it. However, I don’t see much of a point in harping on the choices of others unless they harm others or the animals they are keeping. If someone wants a tiny yellow tang for their 4 ft tank, so be it as long as they make the appropriate decisions come time.Weve got a 65g tank, so the options are slim when it comes to fish that will be appropriate when they're fully grown.
There seems to be a stigma about getting fish with the intention of passing them on to another hobbyist when they get too large. Other than the stress of being rehomed, what makes this so unethical if the animal is relocated before getting so large as to be unhappy in its environment?
Thanks for your thoughts.
I think the overall consensus seems to be that rehoming is okay if, and only if, the fish will move directly to a good home (not through lfs), the fish moves sooner than you think it needs to, and you know for a fact that you can find a taker - so no tank busters or super quirky fish.