Here cookie girl
Sorry to hear about your tank problem. Cloudy water is -very- common. It can have many causes. But what I really think yours stems from is the ich treatment. The medication most certainly has destroyed your biological filter (the beneficial bacteria) and your tank is having to re-cycle again. And there is an nutrient inbalance triggering the bacterial bloom you are seeing. The recent fish loss (platies and 3 guppies) was likely due to high ammonia or other water quality problems such as high nitrites as well.
Your aquarium has to get re-established after the major wipeout from the medication use and absence of fish (no ammonia food source for the bacteria)
The bacterial bloom will eventually wear itself out as the nutrient source becomes depleted. But battling cloudy water is best done with a massive water change. If there are no fish in the aquarium, I'd do 100% draining the water as far as you can and refilling it with dechlorinated water. It is possible you may have to repeat these large water changes because what little water or nutrients leftover in the gravel bed may be enough to trigger another bacterial bloom. But after several large water changes like this, it really should be clear.
If you have fish in the tank then draining the water down to leave just enough to keep the fish comfortable will also work but it may just take a bit longer to get the cloudy water clear again.
Once you pretty much have the tank clear, you'll need to work on cycling it most likely. You said you had the water tested at petco but I wish we knew the exact readings of the water parameters. Going by "It's fine" as I run into so much may not always mean it is. In a cycled aquarium you should get ZERO as a reading for both ammonia and nitrite. And nitrate should be 20 or less.
Maybe your tank -is- cycled though. We never know. And there is simply a bacterial bloom from a sudden nutrient imbalance. But the loss of the platies and guppies you mentioned certainly makes me think of an un-cycled tank with ammonia present.
To re-cycle your aquarium, you'll just need to follow some basic guidelines, a really excellent tool to have on hand is test kits for ammonia, nitrite, and nitrate to see how things are going along....
If you have fish in the aquarium, you'll have to be very careful with cycling your tank. Feed very lightly, moniter your ammonia levels and be sure to change at least 30-50% of the tank whenever ammonia readings go above safe bounds.
If you don't have any fish or critters in the tank, you could either cycle it with 1-2 hardy cycling fish (my favorite is the Black skirt tetra) or fishless cycle which is where you add -pure- ammonia directly into the tank or use flake food where it decomposes and produces the ammonia your tank needs to start cycling.
When you cycle with fish, test your ammonia level everyday and make those vital water changes when necessary. If you see your ammonia and nitrite levels at ZERO after a couple of week you might could add a few more fish. But be sure to test again the day after adding the new occupants. Sometimes the bacterial colony take a little while to get adjusted to the new additions bioload and you may get a ammonia reading. A quick water change will immediately fix that problem and it shouldn't be a big ordeal.
Thereafter, carefully stock your tank again. I'm sure your tank will clear and get straightened out again with time and good care. A very good tip to remember and try to do is to quarantine all new fish in a 'hospital/quarantine' tank of around 5-10 gallons for at least 2 weeks before adding them to your main aquarium. This helps prevent the new fish from infecting all your others and helps keep the illnesses contained should the new fish ever have one. Quarantining is really a must!
Freshwater Aquarium: cloudy/white tank water, cory cats, beneficial bacteria