Controlling Tank Temps

that is pretty warm for tropical fish.

is this tank by a window? direct sunlight will definitely play a factor in the tank temperature. try moving the tank to a different location.

also, what sort of lighting do you have on this tank? certain bulbs (like incandescent) will raise water temperature like crazy. better to run cfl bulbs if the light fixture takes 'screw in' bulbs, or switch to a t8 or t5 style fluorescent tube light fixture. if you don't have live plants in the tank you don't really need to have the lights on much anyway, the fish don't care either way.

lastly, do you have a heater in this tank that is constantly on? malfunctioning heaters are a big issue and there have been a number of brands that have been recalled.

29C = 84.2F
31C = 87.8F

Yes those temps are very high for rummynose.
They prefer 78-82F.

Is your tank covered?
What is the heat source? If a heater, what size (watts)?
Is it in a room where the temp changes often?

I think that you need to open the tank to let the water cool faster and if you have a heater, turn it down.
I have a tank in my room and during the summer I unplug the heater since it will reach over 90F in my tank.
I also open the top and run a ceiling fan to help cool the water faster.

I hope this helps.

Is the room that temp too?? If the room is cooler, I would uncover the tank.

Do you have plants?? If not , turn off lights.

you can try using everyday personal-use fans to blow across the top. use water evaporation to your advantage to cool the water

if you want to get super inventive and impress your friends, you can run a small pump with air tubing into drilled holes in your freezer. use caulk to seal the holes of course. connect the tubing to a small heatsink/radiator inside the fridge, and run a return tube back into the tank. The freezer may seem too cold, but remember your radiator won't be a good heat conductor because it will be made out of plastic instead of the usual - metal. You could rig one up by coiling the tubing around a soda can to maximize exposure to the cold.

if this idea sounds stupid to you, don't worry. It is kinda stupid. But it will give you the results you want

Uhm...drill a hole in the freezer several rooms away and pump water through air line tubing? Really?
LOL

can always do water changes and use cooler water/ just dont use alot colder because it may shock the fish