Tetracycline is an antibiotic. If your bird doesn't have a bacterial infection, you'll just kill the natural flora in the gut required for nutrient uptake and digestion. Antibiotics are just a broad spectrum killer of microbes.
I'd go the route of nutritious food and probiotics if you can get your hands on it.
I did not see your previous post, but have you had a fecal float test done? Any vet can do it, and you don't have to tell them it is a chicken, just say it is a bird. The test should be under 20 bucks for any old cat/dog vet to run, and it will tell you if there are worms or cocci. It will also tell you the type of worms and type of cocci if it is there, and then you can google the right type of chicken wormer/cocciodistant to use to rid of it. Runny poo and lethargic nature often say cocci or prolonged worm infestation to me.
Good luck!