I went downstairs to feed one of my orandas, Quimby, and he seems about the same as usual. Not sick, sort of lethargic (he always has been though) and no signs of illness. I was watching him and he seemed to spasm! His whole body sort of shook and twitched for about 10-15 seconds. He seemed to resume his normal slow swimming after this. What happened?! Is he okay?

Also ammonia - 0 Ph- 7.5 just he and another little oranda in a 55 gallon.

Aquaclear-70 filter.
Water change - 25% every 2 weeks or less.
Aquarium salt added.
Fish are fed Wardley flakes, algae wafers and Manda Fu.
He has some blood streaking in his tail but this is healing after a somewhat prolonged illness.
Quimby has always been somewhat lethargic and used to swim strangely (upside down, float on his head, etc.) but has stopped this since I have moved him to a larger tank and make sure his water is perfect.

Did it look as though he was trying to scratch his body on the bottom or sides of the tank? He may have a parasite issue making him twitchy/itchy.
Could you get the nitrite and nitrate readings for your tank?

No, he wasn't scratching at all. Not near glass or gravel or anything.

Have you seen him flicking his dorsal fish up and down really fast? or doing anything else that might suggest something is irritating him?

We really could do with those nitrite and nitrate readings. Have to rule out water quality problems before anything else.

Nitrite - 0ppm I don't have anything to test my nitrates right now, but everything else is exactly where it should be with frequent and regular water changes. No, he's not flicking his fin or doing anything else to cause alarm.

How much salt do you have added?

1 tsp per 5 gallons

Tablespoon, not teaspoon?
That would be at 0.1%.. I think it would probably be good right now to put it up to 0.3%, so depending on how long ago you added the first lot, add another tablespoon/5gallons, then 12 hours later do that again.