You're cycling, so I wouldn't be too concerned yet. The production of Ammonia (which should be happening at this point in your cycle) can lower PH. So if you're reading Ammonia when you test, this could be the cause and it should balance out naturally as your cycle progresses. For now, I'd start by making sure that you've got good aeration (point one of your return nozzles at the surface so it agitates the surface and creates ripples) for gas exchange, and ensuring that your tank is up to temp. These two things alone can help balance out your PH.
I'd also consider running your lights on a normal cycle if you're not already. If there's anything photosynthetic (coral and/or algae), will consume 02 at night and produce C02 which will lower your PH (this is part of the reason it's important to have surface agitation). During the day when you're running lights, those photosynthetic organisms are consuming that C02 and giving off 02, raising the PH. If you've been cycling without lights, you aren't going to benefit from this process.
While 7.5 is low, I wouldn't go crazy trying to chase PH numbers, especially right now since you're cycling. I'm a pretty consistent 7.8. I pull actual seawater from the ocean for my tank and the on-the-spot reading is 7.8. It's fine. Trying to manipulate things (especially with chemicals) to bump it way up can do more harm than good.
