Alright, so there's several obvious problems here and I'll try to lay them out for you as none are all that hard to fix
1. Get your own test kits. At the moment though, when you take your water to Petsmart make them give you the numbers they get. Don't ever take their word for fine please. When you're there please pick up your own test kits, API brand seems to be the best. You'll at least need kits for ammonia, nitrite, and nitrate.
2. 20gal is way too small for 2 GSPs. They need at least 30gal each and require brackish (high end to full marine) water. Are they in freshwater now? Are you able to properly house both? If not you should give away one or both to someone who can.
3. A diet of dried shrimp is not healthy for them. Please read this article on feeding:
library/feeding/feeding-your-puffers/
4. What is your water change schedule? A little over half the other day doesn't tell us much. Can you please elaborate?
5. Some algae removers can be very harmful to your puffers and your beneficial bacteria. Please do a very large water change ASAP and do not put the Algaefix back in. Have you tried doing a lights out or treating with hydrogen peroxide? Do you have live plants that need the light? If the fish are in FW now then but adding marine salt and turning the water brackish the algae should be killed. There's also no need for you to have an ammonia neutralizer in the tank. Once your fish are healthy again you should remove the ammonia neutralizer. I don't know anything about the water clarifier but you might as well stop adding it until you hear from someone who knows. Your best bet is to remove anything extra from the tank while you sort out these problems.
So, if I were you my first step would be to do a very large water change ASAP. Add your dechlorinator but don't add anything else. If they're in BW then add the proper amount of salt too. Then I would head to your LFS and pick up those test kits and some marine salt if you don't have any yet. Might as well pick up a slew of suitable frozen food. Next I would find a larger tank. If you're going to keep both then go for at least 60gal. In most areas used tanks and equipment are significantly cheaper than what you can find in a pet store.
Hope that helps and please feel free to ask all the questions you need to

We're here to help

It is absurd to talk of one animal being higher than another. We consider those, when the cerebral structure/intellectual faculties most developed, as highest. A bee doubtless would when the instincts were.
Charles Darwin