what is wrong with my betta? please helpp

[klmt]

Ok, so i have a very nice male crowntail betta that i bought about 3 weeks ago. he has been living in a 20gal long that is heavily planted. sharing the tank with him are guppies and swordtails. it is a very peaceful tank. everyone gets along and he was acting and eating great. around the beginning of this week however, he began to act kind of odd. his colors became much more dull, he was less active, he is losing interest in food all together. he just chills at the top of the tank in the plants and doesnt move much which is unlike how he was acting before. i noticed, he is kind of breathing heavier than normal too. what should i do? and what is wrong? would it be a good idea to take him out and transfer him to a bowl and treat the bowl with aquarium salt cause i cant treat my planted tank.

[klmt]

also the water params are perfect. thisis an established tank that ive had no problems with in the past. and i forgot to mention that the temp is between 78-80 F. I feed flake food and ya i think thats it

[Oldfishlady]

That is what I would do-QT and float him in the heated tank for now and start a daily 100% water changes on him-with or without aquarium salt 1tsp/gal

How many plants in the display tank and what kind, how much and how ofter on the water changes and do you have water pram numbers to post, filtration, additives, water temp and how is his appetite and is he able to get to the food before the other fish?

[klmt]

ok great response. thankyou! :) and i just noticed the sticky, sorry about that. but to answer your questions. i do 50% water changes once or twice a month. plant that i have in there are java fern that cover about 50% of the floor and go about 5" up and aquarium sprite which i recently had to trim back a lot because it covered the entire surface almost and was coming out of the tank. no additives, no filter (plants and water changes keep the water in check.) temp is 78-80 degrees. and his appetite was amazing, he got plenty of food, but now he is not interested. and water params are
nitrite-.5 ppm
Hardness-120 ppm
alkalinity- 120ppm
pH- 6.8
nitrite-60ppm (i realize this is a bit high so im doing a water change now, but he is the only fish affected so what could this be?)