The plants will be fine with 2 heaped tablespoons of salt per 20 litres.
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If you are worried about the harlequin rasbora in the middle of the picture, the fish with the pale yellow orange body, it is not velvet. That looks more like a microsporidian infection in the muscle tissue. The parasite damages the muscle tissue and makes it look white or milky in colour. Fish can be like this for a few weeks to several months but eventually they die from it.
The only success I have had treating this infection is by increasing the salt level in the tank to about half strength seawater for a couple of weeks. I used this to treat rainbowfish, which are fine with some salt. However, harlequin rasboras never evolved to deal with salt so it would be risky.
The extra salt would kill the plants so you should move the plants out and treat the tank or move the fish out and treat them separately.
Because it's only 1 harlequin at this stage, you would be better off moving it into a quarantine tank for a few weeks and treating it there. If it survives then put it back in the main tank. If it doesn't, then don't treat the others.
You could try a lower dose rate of salt and it might help. 4 heaped tablespoons of salt per 20 litres of water would be the minimum salt level. Keep the fish in this for 2 weeks and see how they look. This level of salt will affect some plants.
If it is a microsporidian infection, the parasites will be in the tank and could infect other fish further down the track.