Since fish are cold blooded, they will take on invironmental temps.. This also has effect on its metabolism it will run faster at warmer temps. Than fish that evolved in colder regions with (excessive) seasonal fluctuating temp periodes. If these are kept at a constant warm temp will constantly run on a higher metabolisme and it will burn them up sooner shortening their life span. It's not that this is an agonizing torture for them, they simply age a lot faster and reach the end of their lifespan sooner. This super aging can result in weakoning and or catching disseases sooner. Seasonal temp fluctuations likely will also have effect on their hormonal household, since it makes them go into spawning or not. Disrupting this hormonal ballance could also have effect on their health and lifespan. That's a natural cycle for most cold blooded small animals especialy fish.
Thus too warm is rather a relative concept, if you like them to live their lifespan to full potential you need to give them a as natural as possible invironment preferably with fluctuating temps.
This goes for example for most fish indigenious to the northern hemisphere. Naturaly going through a very cold winter season almost shutting down and almost go into hibernation during this cold periode. If these fish are kept in an aqaurium they might only live a fraction of their full cycle potential. The Stickleback is the most popular aqaurium example for this. In natural streams and waters it can live 4 to 6 years in an aqaurium, one that lives longer than 12 months is a record breaker.
Other concerns to monitor if cold water species are kept (too) warm is water polution, since they metabolize a lot faster require a lot more food and thus also produce a lot more waste. Since aqaurium keepres are prone to keep way to much fish per volume, the fish might polute the water sooner than anticipated and thus require a lot more water changes to prevent toxic ammonia build up.
@zozo thanks for the insight! There is a lot of food for thought in your response. I don't think I'll be able to drop the tank temp too much without a chiller but I could probably get it down a couple more degrees. @72 deg farenheit I shouldn't have any adverse effects on my plants should I?
Interesting. WCM are extinct in their native habitat in White Cloud Mountain, China due to industrial pollution, but has no trouble naturalizing outside China in many places due to their cold hardiness. They are called poor man neon tetra IME, they are superior to neon in hardiness, school tighter, males display interestingly to females, and breed easily as adults don’t eat their young. The reason I am not keeping them now is that they are too tiny, and can easily be bullied or eaten by my bigger fish.
These were the very first fish I kept (not counting local cought stickleback and the likes in buckets and dishwashing bowls alongside all kinds of water-beetles, frogs and salamanders - sorry, but yes I've been there, killing off a few, too). The "White Cloud's" were really fascinating in behaviour, quite easy to care for, nicely colured and allways in good health - and important to a very young kid: very affordable.
Kept on their own, they did not pray (significantly) on their eggs and fry. As I recall, the juvenile fish displayed a quite bright, green stribe - allmost as irridescent as the colour of neons. Not surprisingly they seemed to appreciate quite strong flow in their tank and thrived at standard room temperature ( 18 - 20 Celcius) and plain tap-water in regular waterchanges.
A highly recommendable, little fish.......... (come to think of it, it may be time to get back to the simple joy of these little gems).....