Shaking Dwarf Neon Rainbow

Hi all, I have a problem in my tank am need a bit of advice.
The other day a lost a dwarf neon rainbow, he seemed to lose all appetite and lost his colour, I did a quick water change but come the morning he was dead.
Now I notice another one is shaking violently as if having an electrical shock.
The shaking is sporadic his colours are fine and he eats really well.
I am worried I may have a parasite?
See the attached which shows a blemish on his right flank but it is so small it may just have been damage from a boisterous feeding frenzy.
The shaking seems new today, but it doesn’t look good.
He doesn’t seem to rub against anything just violently shake befor righting himself and swimming off
Does anyone have any ideas?
90 litre tank, temp 26
Water parameter are
Gh 180
Kh 180
Ph 7
Nitrite 0
Nitrate 20 max.
Full planted tank diet of algae wafers, flake and tubifex.
Have done a further large water change today, treating water with API Stress coat+
Any help gratefully accepted!

Can we get a video of the fish shaking?

If rainbowfish ever get sick or do funny things, clean the tank and add salt.

Wipe the inside of the glass with a clean fish sponge.

Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn.

Add 2 heaped tablespoons of rock salt, sea salt or swimming pool salt for every 20 litres of tank water. Keep the salt in there for 2-4 weeks.
If you do water changes while using salt, add salt to the buckets of new water before adding the water to the tank. Then the salt level in the tank will remain constant.

Thank you!
Link to video is here:
Shaking kicks in around 40 seconds, excuse the lengthy video...
I will do another large scale water change in the morning as I did one this morning also.
Will the salt be ok with plants? Java fern, crypto’s and vallisneria
Thanks again for the reply, very much appreciated

The salt level I recommended is fine for shrimp, snails, fish, plants and filter bacteria.

Cheers.
First job of the morning.

really appreciate the advice