you can freeze water bottles and put them in sump. keep a few and rotate them from freezer to sump.
the easy way as mentioned run ac. i keep mine 74-76 with a fan, temp hasn't gone over 80.5 thats with 3x250w halides.
that y i told u t get another t5 that way u can shut down ur mh on really hot days but for now u can put fan facing the tank and leave the ac running most of the time when ur tank is on trust me i know a trick or 2 about dealing with heat i been running mh my whole reefing ventures
If your tank is small, get a cheap fan hook it up to apex or on a simple timer, should do the trick. Mine been running for 2 years no problems. But when it gets really hot this summer an AC is a must! Or chiller if you can afford it/.
get a a few 1 liter flat jugs. At least 2. Fill with RO water and freeze. then I alternate one for the other on hot days. I place them in the drain side of sump and i off my refug light in the day time and run only at night in the summer,
Also do your maintenance on your pumps the gunk and stuff will cause them to run hotter adding a few degrees to the mix