Question about fish scratching on gravel?

[Scy64]

I've noticed that a small koi I've got in a hospital tank has been scratching against the gravel substrate at the bottom of the tank recently. He's currently being treated for a cloudy eye, and I want to make sure that there aren't any other unseen problems going on.

Water paramiters:
Ph: 7.4-7.5
Ammonia: 0ppm
Nitrite: 0.25 ppm but lowering
Nitrate: 40 ppm but was higher before water change/water conditioners

Fish: One 6 inch koi (counting its tail)
Plants: None (should I add a few living plants from the established tank over to this one? Medication I'm using claims it won't harm living plants, but I wasn't sure if it was worth the risk).

This is a fairly new tank, so I'm sure the cycling process wasn't totally finished when the fish went in. I used gravel and water from my already established tank (where the fish normally lives), but the biological filter was new and without bacteria. The tank has been running for a good week now, with about a 50% water change at least once a week (or two 25% changes twice a week).

I'm *assuming* that right now, it's the water causing the scratching behavior. I see no external problems other then the eye problem (which is being treated). I'm currently using water changes as my main attack against the nitrate/nitrites for now, until the tank really stablizes.

Is there anything else I should be looking out for? Anything else I should do?

[emc7]

I'd go with artificial plants and add salt.

[Scy64]

Sounds good, I'll give the salt a shot (I've got one artifical plant in there already).

Is there a spefic brand you'd recomend? I've never used salt in a freshwater aquarium tank before.