I have a 57 gallon tank that I have had at around 72° for a couple of months now, and this has been working very well for me.
This is an ultra-low maintenance tank. I gravel-vac and change about 1/3 of the water every couple of...um...weeks (more like months), and I feed the fish about 2-3 times a week. It's an open-top tank, so I have to top off the tank about once a week with about 3 gallons of fresh water. Every so often I will trim the plants and clean the front and sides of the tank. That's pretty much it. Sometimes days go by when I don't go near the tank.
This probably sounds like I am neglecting the tank, but it is actually a very healthy system and has been going strong for quite a while. I just like to things to be slow because I don't have much time for maintenance. (I could lie and blame it on work or family, but it's really the small reef tank I started last winter.
) So I don't have CO2 or tons of light because I don't need my plants to grow a foot every two days. I feed sparingly to avoid a lot of mess and discourage spawning.
Having low water temperature seems to help slow things down as well. That's nothing new, of course, but so far I have only seen advantages and no ill effects whatsoever. Plant growth seems to have slowed down, which is great when you mainly have swords, microswords, and crypts. Algae growth seems way down. I have green spot algae that grows on the glass and an insignificant amount of BBA on the filter and pump. In this slow tank, it's easier for me to remove the algae every couple of weeks than to figure out and manage the root cause. How often do you hear that?
The fish don't seem affected at all. I have small schools of diamond tetras and cardinal tetras, as well as a bunch of platies, otos, and an old big pearl gourami. None of the fish have shown any signs of discomfort. The platies (supposedly all female - at least one must have been pregnant when I bought it) and diamonds have offspring regularly, and everyone else seems happy and healthy as well. So I really wonder why I would ever want to go up to higher temperatures again.
Anyone else have experience - positive or negative - with low-temp tropical tanks?
By the way, with the exception of maybe the cardinals, all my fish are known to be fairly tolerant when it comes to temperature. I wouldn't try this with discus, rams, or other warm water-loving species.