Help! I think my green spotted puffer is sick!?

I have 2 gsp One has a very large appitite, very dark spots, a bright green dot and is very active and healthy. The other one (the sick one) has always been on the timider side and has always had natrually lighter spots. Lately i have noticed the the puffer has had very little white dots on the end of its dorsal fin but it does not look like fin rot. He has been eating fine.Tonight however, I trengwent to turn on the light and i saw that she was just sitting there suctioned against the side of the filter almost has if he did not have enough strnght to swim away. I pushed him off with the net and he swam away but looked very pale and swam slowly on the bottom of the tank. I crushed up a few pwices of freez fried plankton and put them in the tank. After a few minites he began to eat and seemed to get stronger and his color came back and was pretty dark again. He then seemed more hungry than ever and was eating every peice that he could find. But his belly did not look as full as yother puffer. He looked hungry after it was all gone so i put in some seaweed gel because he usually like to take a few nibbles on that. He is now swimming around but i noticed a stringy looking poop hanging from him. I checked the water and everything is good. I am planing to siphon out the tank for the first time tmr ( i only have them for about a month and a half) and i will also feed them blood worms and i will put a crunchy vitamin block to file down their teeth. I have a 10 gallon tank but they are still small and i am planning to eventually get a larger tank. I have gravel and they are in freshwater right now but i will start adding salt soon. they also did not have black bellies but i noticed some stress lines but they went away. thanks in advance