Finrot wars...Please help!!

[aqua001]

So, I have a betta who is continuously losing fins and after that, they rot and more falls off. He seems more lethargic lately and also stressed

Note: He has tank mates.

Housing
What size is your tank? 10 gal
What temperature is your tank? 76-78 degrees
Does your tank have a filter? Yup
Does your tank have an air stone or other type of aeration? no
Is your tank heated? yes
What tank mates does your betta fish live with? 6 penguin tetras

Food
What type of food do you feed your betta fish? Betta Bites
How often do you feed your betta fish? Twice daily, 1 pellet each time

Maintenance
How often do you perform a water change? once a week
What percentage of the water do you change when you perform a water change? 25-50
What type of additives do you add to the water when you perform a water change? Aqueon water conditioner

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Ammonia:--
Nitrite: 0.00 ppm
Nitrate: 0.05ppm
pH: 8.0
Hardness: 120 ppm
Alkalinity: 120 ppm

Symptoms and Treatment
How has your betta fish's appearance changed? Stress strips, black slashes on head.
How has your betta fish's behavior changed? rather lethargic, hides more often.
When did you start noticing the symptoms? About a week ago
Have you started treating your fish? If so, how? no
Does your fish have any history of being ill? Yes, fin rot but he healed.
How old is your fish (approximately)? --

[aokashi]

are the tetras nipping your betta? could have driven him to paranoia...

[lvandert]

I get my boys over fin rot with aq salt, stress coat and IAL. Generally gets them growing fins back withing a week. I would keep dosing the aq salt (for normal parameters, sick parameters are different) as a precaution. It might keep him from getting it next time.

[aqua001]

Yeah, I think the tetras were driving him crazy. I will get aq salt ASAP. Now,another question is how does tetra poop like like? I have found short stringy pieces all over the tank either red or grey in colar. I know for a fact my Betta is not this messy. Do you think this is healthy tetra poop or unhealthy Betta poop?