'Seasonal' temp fluctuations

  • #1
Hi forum!

I've been thinking a lot lately about the use of heaters in fish tanks, especially in relation to the cost of electricity at the moment.

I'm restarting my 'fish room' (bedroom) and I'm trying to keep costs down by keeping fish that like cooler waters, using air powered sponge filters, even using ambient light instead of direct lighting on some of my tanks.

I've been doing some research online about this as well, and there's a train of thought that suggests we might be keeping our tanks too warm?

Given last years heatwave (I'm in the UK) and the likelihood of upcoming high temps for longer than usual, and the fact that keeping temps down in the summer is a challenge rather than the other way around, I'm wondering about the wisdom in keeping a heater in the tank set to the lower end of the temperature scale for the fish (I get my temps from seriouslyfish, sometimes fishbase) for winter, and allowing the temp to rise naturally with the temp changes in summer.

I don't think this would work with warm water fish, of course, but for the majority of tropical fish we keep, also considering their origins (eg, farm bred, captive bred, not so much wild caught) it seems to be that we can lower our heater temps and also our elec bills without detriment to the fish, which is the most important point.

Positives also include longer lifespan and less aggression at lower temps, although I've also read fish kept on the cooler side don't grow as large.

Just wondered what the hobbyist consensus is on the situation, and if anyone, especially in the UK, has moved to low-set heaters, or no heaters at all.

Thanks Fishloreans!

  • #2
For cool water species this is completely fine. As long as you’re not talking about rams etc I can’t see this being an issue. Bear in mind fish metabolism slows down at cooler temps so feed a little less.

just research any fish carefully to make sure it doesn’t need tropical temps.

Also it’s cheaper to heat a room than lots of tanks. Although at the minute who knows ;)

  • #3

this is actually what I'm planning to start this year. Warm in the summer, cooler in winter.
My rather tropical tank will then be allowed to fluctuate between 21°C and 27°C during the year, the shrimp between 18°C and 27°C.

Of course depends on if the fish can take it.

I'm already simulating the day lengths (longer in summer, shorter in winter), sundawn and sunset and cool rain showers somewhat, so why not go all out.
  • Thread Starter
  • #4
Blacksheep1 thank you for taking the time to comment :) Yeah, no, I didn't include things like rams and discus who need things nice and steamy. But I was also considering expanding the list of fish that can go cool past things like WCMM and goldfish, etc, to maybe things like common tetra species, barbs, that kind of thing.

Like, when we say 'tropical fish', how tropical is tropical, you know?

Yeah! Feeding less helps with cost too, and with pollution in the tank I'd assume. (Note as well that I'm not interested in breeding, so things like nourishing pregnant fish and fry aren't a massive concern for me)

I'm looking at a tank of glassfish - I'd ideally like to get parambassis lala, but I think parambassis ranga is more common, if they're ever even labelled correctly - and their temp range is 20-30C; that's a 10 degree difference. I wouldn't need a heater to keep the tank at 20/21 (that's around what my current test tank is, although it's much smaller than the tank I want for my glassfish, so more experimentation needed on that) and I'd imagine I'd get warmer temps from the heat from the light and then warmer still in summer - and there'd be a natural gradient between the surface of the water and the bottom of the tank, you know?

But they also share a home (Lake Inle) with sawbwa resplendens, the asian rummynose, which I was looking at. This topic actually came up because I wondered if I could keep a group of these with the glassfish, and I knew the sawbwa can be kept heater-less as they're range is 18-23 (off the top of my head)

You could argue that temps fluctuate even in a body of water like a lake, depending on where in the lake the fish gather, depths, etc, but still. Could glassfish be kept without a heater given the above?

I'd totally heat the room, that would be so easy, but I've got dysautonomia and my body goes crazy with fainting and dizziness and things even in warmish rooms - I'd die in a room that was heated enough for the fish, unfortunately. Thank you again for taking the time to reply, I appreciate it!

Zer0Fame That sounds really interesting, are you logging it anywhere? I'd love to see. How's it going so far? What are you keeping at those temps?

I'm planning on getting a channa andrao, and that species needs the seasonal fluctuations, which also sparked off a 'hey, maybe this could apply past just a couple of species'

Yeah, whether or not the fish can thrive in those conditions is what makes me nervous, I'd hate to lose any because of something I did for my own reasons, you know?

I'd love to hear more about your project!

And thank you kindly for taking the time out of your day to post a response, I appreciate it :)
  • #5
Ooh yeah don’t heat the room then !! Eek. Medakas are an option. Just not oryzias woworae they do need tropical temps.

I’m not 100% sure on the Asian rummynoses but regulars do really appreciate warmer temps and will end up having various issues if kept cool, I’d research more if the Asians are different though.

I think ‘tropical’ depends on the individual species . We say corys are tropical , but pandas do well cooler and sterbeis like it warmer. Im not sure on the glass fish , sorry about that .
  • #6
are you logging it anywhere?

Not really no. :(
Though my interest in getting a tank computer is very high so it's tempting. xD
I want to do far more even ... as in constructing a small LED and simulate the moon phases.
I already have a different flow in the evening and the night to simulate different flow speeds of the water.

I'm keeping Pseudomugil signifer and Microdevario kubotai. If it wasn't for the kubotai, I would even let it drop way more. I like to keep my rooms at max 18°C in winter anyways so I would easily let it drop to there. However, the kubotai have a 20°C minimum, so I want to keep it at least 21°C for some safety.

The fish are at 22°C at the moment and are doing just fine! I'm curious to see how the behavior will change when it gets very warm. Max range for the signifer is 15 - 28°C and 20 - 27°C for the kubotai. So I just guessed that 21 - 27°C will be nice. :)


  • #7
My rather tropical tank will then be allowed to fluctuate between 21°C and 27°C during the year, the shrimp between 18°C and 27°C.

I'm not saying you should, but you can keep shrimp a lot cooler than that (at least you can with neocaridinas). I leave some outside in tubs over the winter. The top can be covered with ice for several days, and as soon as it thaws I can see the shrimp out and about like nothing happened.
  • #8

unfortunately that's the temperature I have in summer with a fan cooling system, so without active cooling I can't have them any cooler. :D
  • #9
Seasonal fluctuations are fine as long as it's within their natural range. For some equatorial species that's quite a narrow range. For others, especially into the subtropics, it can be quite wide.
  • Thread Starter
  • #10
Blacksheep1 lol, yeah, I won't. I REALLY want some medaka - I wanted to do a patio pond, but you can't keep them outside in the UK. That doesn't mean I'm not going to keep some inside though, although I've heard they do better outside? Maybe something to do with the sunlight?

Yeah, the similiarity lies in their red noses, but they're different species.

Exactly. I've lived in a tropical country, they aren't hot all the time, especially in higher areas, especially the mountain rivers, they are FREEZING.

That's okay, you're so sweet! Thank you for taking the time to reply :)

Zer0Fame I'd be interested in some data from people, see what temps they reduced to and how the fish did. I can get that it might be a bit of a chore, especially with the other parameters you're playing with. I couldn't do it, for sure!

Oh, nice! I was looking at p. luminatus, but I'm not sure. I LOVE kubotai though I haven't kept them yet, especially in blackwater/tannin'd water, I can't believe how much they glow. I don't know if I'd experiment with a heaterless/low heater tank with fish like that though, you're right.

Nice! And thank you for responding! :)

GlennO Yeah, from what I gather, the general recommendation is to keep fish /the heater set in the middle of their range, but I'm wondering if setting the heater to their lowest temp and then letting the temps fluctuate naturally wouldn't be sustainable. I've heard 'that temp is at the lower end for that fish, so it won't work' a few times when people have asked about compatibility for fish, buuuut, I wonder if that's as much as an issue as it seems.

Similiar to PH in water - okay, it's an issue for things like super soft/hard water species, and wild caught specimens, but if the fish have been bred for a few generations in a local PH, does that even apply?

I suppose that keeping a low heat/heaterless tank would apply more to the subtropic fish than the ones with a more restrictive range?

Thank you for taking the time to leave a response, I appreciate it! :)

ProudPapa nice to see you!


  • #11
You’re more than welcome. Sorry I’m not of any more help on the fish you actually want ! I’m uk too. I think it’s reduce any chance of them getting into local water ways more than anything. I have the daisys so mine are indoors but I do keep gold wcmm outside in the summer. They come inside over winter and live with the goldfish. The temp swings they’ve had last summer and did really well surprised me.

Such an underrated fish.

It’s also not that easy to get Medakas , apart from daisys, unless you want eggs.
  • Thread Starter
  • #12
Blacksheep1 Not at all, it's been lovely to talk to you

As far as I understand it, people selling medaka do so under a license that states they can only be sold to be kept indoors, and that sellers are supposed to be passing that on to customers. It's not -illegal- per se, but it's also kinda unallowed.

I'd love to keep WCMM outside come summer, IDK if I've got the energy to do it though, stupid disability. Those are gorgeous! I do like the normal ones more though, but wow.

I've seen some available on the bay of E! I don't know if I want to get all the same colours or just one or two - the black ones are gorgeous.
  • #13
Some subtropical species will do well with the seasonal variation in temperature, and may even need it - like the Gymnogeophagus species for example. Others from southern Argentina and Uruguay also like certain Apistogramma, tetras and catfish species should do well too. Species restricted to the Amazon basin or Orinoco river drainage where temperatures are quite high and constant throughout the year would probably not fare so well. Species found in Southern Africa (eg Pseudocrenilabrus philander, some mormyrids etc) should also do well with seasonal fluctuations, but not those from the much more stable rift lakes or Congo basin. I am not as familiar with Asian species but there must be a number of subtropical species there as well.
  • #14
GlennO I've heard 'that temp is at the lower end for that fish, so it won't work' a few times when people have asked about compatibility for fish, buuuut, I wonder if that's as much as an issue as it seems.

Similiar to PH in water - okay, it's an issue for things like super soft/hard water species, and wild caught specimens, but if the fish have been bred for a few generations in a local PH, does that even apply?

I suppose that keeping a low heat/heaterless tank would apply more to the subtropic fish than the ones with a more restrictive range?
I wouldn't keep them at the lower end of their range permanently, but if you're doing it seasonally that's just a reflection of the natural variation that they would experience.

Captive bred fish are generally more adaptable than wild caught fish though I don't know if it applies as much to temperature as it does to some other parameters. I feel that the 'captive bred' reasoning is sometimes used as an excuse to keep fish in less-than-ideal conditions. You can't completely override millions of years of evolution with a few generations of captive breeding.

Yes generally speaking sub-tropical fish would be more suitable for heaterless tanks. It's just a matter of whether the room temperature stays within their natural range.
  • #15
You could change the course of evolution in a relatively short time frame, but only if you selectively breed on specific variables. Question is if this has been done for temperature.

Would breeders use temperature as a selection criteria for a market that isn't asking and given the fact that temperature itself relates strongly to spawning and breeding.

And just because breeders likely keep temperatures at spawning behavior mode year-round, doesn't mean fish have evolved to these circumstances.

So the species' natural temperature conditions probably serve best.

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