Hey svirid!
Well, I guess the good news here is that what you are experiencing is a very common problem for a lot of us. The other good news here is that 84 degrees (F) is not completely deadly for most livestock - but obviously not ideal.
Anyways, it might help to know a little bit more about your situation...such as:
What type of thermometer(s) are you using to keep track of the water temp?
--- The reason I ask this is because I found it a bit odd that you cannot lower your water temp, even if you place ice in the water since this should make at least some impact if used in sufficient amounts...so I am wondering if you are using one or more of those 'stick-on' thermometers that you place outside of the tank because they are notoriously inaccurate and are really only measuring the temp of glass versus the water itself; hence, the temp of the room may be throwing you off.
Where is your thermometer(s) located in and/or on the tank?
-- Again, since the ice didn't help you at all, I am wondering if your thermometer is only measuring the very top of the tank and/or is under a light which could be warming the thermometer itself. Personally, I would always suggest using multiple thermometers, located in a few spots throughout the tank in order to garner the most accurate water temp overall.
Are you using a heater on this tank?
-- Yes, I completely realize that this might sound like a dumb question since it would seem like common sense to turn the heater down in your tank to lower the water temp, however, I also realize that many hobbyists, including myself, use a heater set to kick on to prevent the water from becoming too cold. That being said, if you do the same thing, it may be worthwhile to ensure that the heater is set correctly and/or is operating correctly.
Please don't misunderstand me though....I am not trying to suggest that your water temp is high because of something you have or haven't done, nor due to an equipment defect since this can happen even under the more perfect conditions, in which case a chiller may be the best thing for you. Obviously, while it can happen to anyone, it makes a lot of sense to say that smaller tanks are probably more problematic in this regard simply because there is less water to warm or chill, so this is again, very common. In fact, my first saltwater tank was a 90 gallon and even though I set it up in the winter, I also struggled with some water temp problems, which were compounded as summer approached - however, now that I have a 125 gallon tank, even though I am using the exact same equipment I had used on the 90 gal - even adding Mh lighting which I did not have before, my heater is almost always needed to maintain 78 degrees or so rather than only kicking on when the water temp drops like it used to. So, just in the addition of about 30 more gallons of water, my temperature issue became extinct.
. Evap rate will go up, but the temp will go way down with the addition of fans
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