Fish unable to steady itself

[Scott1980]

Here's the tank Descriptions.
20 Gallon Freshwater Planted Tank
(Eco-Complete)

DIY CO2

Fish Contained:
5 High Glow Tetras
2 Angelfish (Small right now)
3 Black Skirt Tetras
3 Apple Snails
1 Otto
1 Female Betta
1 Butterfly Gobe
1 Siamese Flying Fox (I Think)
1 Clown Pleco
1 Dwarf Neon Gobe

Water Parameters
Ammonia 0ppm
PH 6.5
Nitrate 20 ppm
Nitrite 0 ppm

Filter is a H.O.T Magnum

I've had the tank for 4 years, but moved twice. It's been in it's new setup for 9 months.

Yesterday morning one of my high glows started swimming wierd and couldn't really control thierselves. I took it out, put it in a different container, added epsom salt. I also did a 25% water change to be safe.
This morning, I wake to find the one high glow still floating around and the gobe upside down on the bottom plus my female betta on the bottom. Now the betta is really really old, so I wouldn't be suprised to lose her, but it's funny to happen now. If the Epsom salt thing didn't help, could this be bacterial or parasite and if yes what should I use. Most of the fish have been in there for months or years. The gobes were the most recent and were added in the last 3 weeks. First time I've ever had problems in that tank. Thanks for any suggestions.

 

[Scott1980]

By the way the temp. is 74 degrees

 

[Devilishturtles]

Did you add the epsom salt to your main tank, or yout QT container?

Internal infections often times show no signs externally, and cause swim bladder issues. If your fish are lethargic and laying on the bottom of the tank, however, it sounds like something else is wrong. How often do you do water changes?
Have all of those fish been in there for the 9 months? (besides the gobies)