I have a 160L tank with a fluval 305 external filter, In the tank are a fantail I’ve had 2 years, Oranda and Ranchu I've had for 18 months, a Ryukin I've had a year and two zebra Danios I’ve had for 18 months.
All fish are usually very active, fat and healthy looking. I've had a problem with High nitrate recently and have been using Nitrate remover in the filter, which helped a bit.  I read pellets can cause problems and  I have stopped feeding them these since last Sunday,  the nitrate level has dropped dramatically.
The Ranchu recently has started hiding and wedging himself behind things usually so he is vertical in the tank. I noticed a white small raised spot on his side last week but it hasn't got any bigger. He swims around the tank erratically when he does come out for food.
The Orlanda had a funny turn over Christmas which lasted a few days where he lay on the bottom of the tank and when he moved he is on one side. He has now gone back to doing the same thing since mid last week and although he is attempting to eat the flake food when it drops down I am not sure he is getting any. I also stopped the pellets because he was like this as I thought it may be the food.
The water parameters are:
Nitrate =<0.1
Nitrite =<10
PH = 7.1
On the weekend I removed the carbon filter and added about half the recommended dose of general tonic to see if this would help, I replaced the carbon filter yesterday as I was worried about the water not being filtered.   
The fantail seems to be either worried about the Oranda or bullying him as he keeps swimming alongside him in the tank and nudging him along. I have read about clove oil euthanasia and would consider this if they are in any pain or distress but I would prefer not to have to do this.
Any advice would be greatly appreciated.
thanks Ban

Noss, I believe Ban's Fluval 305 External Filter does 1000 lph.
With goldfish the reccomendation is to aim for 10x the tank size, so your tank is a little under filtered.

If you are having issues with the nitrite, please check your fish's gill's, they should be a nice crisp red colour, if they are brown, this is a symptom of nitirte poisoning, which can be treated.

With regards to the raised spot, it is the one spot? Is it fuzzy, or smooth? Are there any other sores or redness on the fish?

Let us know how you get on

Thank you for the the replies, my apologies for the abruptness of the query and the mess up with the information, I did get the nitrate and nitrite mixed up. luckily the tank has never had a nitrite problem, the test is usually clear and never more than a slight hint of pink. I was a bit stressed yesterday worrying about my fish.

The Fluval 305 specification is for 300L tanks and pumps 1000L per hour. I deliberately bought a pump bigger than advised as I have had to change to a bigger tank twice in one year already (from 15L bi orb to 38L tank to 160L tank I have now).

I do a 20L water change and sand hoover once a week with a glass clean. The filter I clean once a week and rinse the carbon filters and foam in clean de chlorinated water and the bio media in the water from the filter.

The spot on the Ranchu was like the one in the picture. I have just had a look at him and the spot has gone but thee area is still a bit raised although his scales are smooth over it. He seams to be swimming a bit less erratically although he is still hiding most of the time. Hopefully he is not in serious trouble.

The Oranda is still on the bottom of the tank and not looking happy so I am still worried about him. Would you advise another general tonic? All fish look outwardly healthy.

Thank you again for the replies.

swishkee...that's the baby , js  about 14 mo old, his tank mate (same age ) is twice the size