Betta Fish: Sudden Illness

In these images is Hydra, my beloved shortfin betta. When I got home from work yesterday, he was like this on the bottom of the tank. I quickly checked water perameters:

Ammonia: 0
Nitrate: 5
Nitrite: 0
Hardness: 75
Chlorine: 0
Alkalinity: 300
PH: 7.8
These results are stable, and nothing fluxuates.
(My ammonia test kit is new, but my testing strips are old. I think some of these results are faulty. New ones are coming later today.)

Other info:
-It is a 10 gallon tank, fully bioactive with live substrate, plants, etc.
-A sponge filter is the primary filter of choice
-I do 20% water changes every 3 weeks or so.
-Friends in tank: 8tetras, 8 pygmy corydoras, 2 dwarf frogs, 2 nerite snails, 1 mystery snail.
-I was going to upgrade them into a 15 gallon last night, but was put on hold due to Hydra's condition.
-they do have a grow light. (Its on the white spectre, as it's made for terrestrial plants. A proper aquarium light is here and waiting to be used once the 15 is up and running)

Symptoms:
-A few days ago his tail looked funky. Completely normal except this one bit in the middle that kind of jutted out.
-No signs of stress and normal behavior until yesterday, when there had been rather sudden weight loss, signs of fit rot, a protruded abdomen, and laying om the ground.

Where he is now:
-1 gallon hospital tank (that im suprised holds water). Tank has fake plants, small sponge filter, and a heater. (I stole it from my betta in my 5.5. He'll be fine for a while lol.)
(My room temp is about 75 btw. I have a lot of reptiles with heatlamps)
-temp in hospital tank is stable at 82. It has been dosed with aquarium salt andParaguard.

Thank you for reading all that. I'd like to know what has happened to my baby boy, as well as any other info or advice on how to get him better.