Now that you are running salt water, is anything else in there that was in there when you had fresh water? Like sand or rocks?
You may not have enough air in your system. I looked at your system again and it doesn't look like you have any air stones. Until you get a skimmer, you need air stones to saturate your water with oxygen. If you put on a skimmer then you won't need the stones. If you don't have enough air in the water then your specifig gravity will read low. Once you raise the air in the system, your SG will raise up too. Before you add salt (if you need it) make sure you take care of the air problem.
Once you get your air and SG correct, then you need to check the hardness of your water. If your hardness is too low, that could cause your pH to be low. I use an additive called Kent Tech CB to control my water hardness. This is a two bottle formula that you mix to add calcium and minerals to the water to help with the buffering. Adding this will raise your pH up to 8.3 where it should be. When I mix up fresh sea water, my pH is always low until I add the Kent formula.
This all takes time, patience, and bunches of money. Take your time. It was over a month after I set up my tank until I put some fish in it and even then they were just damsels. I've had mine set up for 2 years and am just now getting to the point to where I want to put corals in it (although this is longer than is needed).
Get all of your water tests in check (temp 76-80, SG 1.023, pH 8.3, lots of air) then wait for your cycle to finish. Add more LR as soon as you can and even add dead regular decorative rock. This is cheaper than LR and after a few months with LR in the tank, your decorative rock will become living too. I can't tell the difference between the LR and the dead rock in my tank now. Now it's all living, and it helps to filter the water.
You should see a rise in ammonia, followed by Nitrite, then finally Nitrate. Once your ammonia and Nitrite fall naturally, then you can considered your tank established and ready for more fish. Let your Nitrate level be an indication of a water change. Once your Nitrate starts showing up, change your water, but only after the cycle has completed.
Add a skimmer anytime you can. The sooner the better. You can feed ozone into the skimmer also and this will make your water COMPLETELY crystal clear. I use ozone and I have a thread discussing it if you want more info.
Always add just a few fish at a time and keep a good watch on your levels when you do. If you see a rise in ammonia or nitrite and it doesn't fall shortly after adding your new fish, then you have too many fish for your filter. Either get rid of some of them or upgrade your filter system to something like a sump wet/dry system or a refugium. When you add fish, wait for your levels to normalize before you add anything else.
Later, add some shrimp and snails or hermit crabs to keep your sand bed clean and your algae under control.
:joy:
:happyfish :happyfish