That's normal for what you did. Ph decreaser causes all the dissolved minerals to clump together. The water gets cloudy and there is 'dust' all over everything in the tank. Its not harmful to fish per se, but a large drop in ph is. I assume you are cycling fishless though, or you would likely have asked.
Here's the deal on ph adjusters. Stay away from them. Yes, they are in all the stores. Yes, the books tell you what ph your fish 'need'. However, your water is your water. It is also the water of your local stores. This is good because the fish will already be acclimated to a high ph when you get them. Really the only time you need specific ph is when breeding certain breeds or keeping fish that are too difficult(and expensive) for a new fishkeeper anyway. You are hopefully years from boredom with all the easy fish out there that will do just fine at 8.2.
I will assume that your water is hard. While you need a chemistry degree to truly understand the relationship between all these water factors, read up on here until your eyes cross from information overload, and remember that hard water is also usually very ph stable, which is a thousand times more important to your fish. One word of caution, however, nitrates/nitrites are more dangerous at high ph, so keep up on those water checks/changes and get dead fish out pronto.
Whether you have fish in your tank or not, leave it alone for now. The cloud and dust will disappear as the effects of the chemical dissipates and the minerals will redissolve. Keep the jar of ph decreaser. Use it on a cotton ball to clean the mineral scale off your filter tubes and splash guard in front of your lights. Keep it away from your water though. Use it just for cleaning.
Ask what your store's ph is where you will purchase your fish. Call, so you know. If its very different from yours, get back on here and we'll tell you how to acclimate the fish.
Did I get anything wrong everyone? If so, pipe in so newbe and I can both get it right.
Newbe, as you can see from my signature, I have all kinds of fish that prefer lower ph, yet mine is 8 also. My angels came from across the country, so I discussed this with the breeder in advance. I wanted to know the water parameters so I could be prepared for any acclimation needs and whether the fish would do well if there was a big difference, as they were quite young. Turns out his water was hard with a high ph just like mine. Not only do his angels thrive, they freely spawn. Well, mine are thriving all right. They are huge and gorgeous.
What kind of fish are you planning on?