Not easy for me to answer this because your far north, the temps. don't go as high as 97 & high humidity like it does here. One thing for sure, it isn't summer yet, so most probably it will get hotter, the problem your facing is the room will probably get warmer, the chiller will up that more. If you cool the room the water will eventually level off If the conditions were the same day & night but you don't run your lights day & night so it changes, also the sun warms up everything during the day more, it take time for the room temp. to equalize with water temps. Like Don said it doesn't happen fast. Sorry I don't have a cure, how much of a day & night change I can't say, you will eventually get into the heat of summer & see. No doubt the AC will help but it will swing, I had 130g with one MH & VHO's & I have central AC, never could stabilize the temps. within reason, the AC eventually cooled the tank some at night but the lights warmed it up a good bit during the day, even with a bunch of fans. What to do? It will get hotter! A chiller will heat the room up but will keep the tank cooler but you will need fans to blow the added heat away & even then I thing you will need to leave a window open or add the AC. I don't want to give bad advise & make you wish you did the other thing IMO I still believe your best option is both. Maybe get one of the two & see how it work out.