from what i heard from other reefers, its best not to keep near windows as the heat from the sun can be nasty.
what i can suggest is get a temp reader ($5-$10) can place in tank to monitor temp using battery, observe whats the degree at afternoon when the sun is high high up.
if its' over 32 degree, i suggest you re-locate it, getting a fan might help if you dont let the temp rose too much
2 cents from me and hope it helps.