Do lots of research before getting any live plants...they can have a lot of special requirements.
Who suggested you add baking soda?? That could cause problems in itself...I wouldn't add any. Aquarium salt is suggested for some fish, and not for others...some people swear by it, but I haven't ever added any to my tanks except when I had mollies.
Do you have a test kit that measures in ppm or mg/l? Nitrates should be relatively low in the tank, and if they're high its a sign that you're not doing enough water changes (most suggest at least 30% every two weeks, most do more like 20-30% once a week) OR you've got too many fish for that size of tank (or not enough filtration for the amount of fish you have) or that you are overfeeding a LOT. Usually its reccomended that you feed once a day, about as much food as the fish will eat in about 5 minutes. You have to be sure to feed fish (like the pleco) what they need, and at the time that they'll eat it. Most plecos are more active at night...so the best time to feed them is right before you turn the lights out at night. If you mix up their food schedule it helps too. Get some bloodworms or hatch some bbs...cut up some cucumber for them or some lettuce for the pleco.
I dont know if it applies to fish...but temperature causes a lot of things to be more or less active/aggressive. Has your temperature been steady? What temp do you keep the tank at?
What size tank is this and what fish are in it?