Hi, i have a 60 litre tank, with 5 guppies, some cardinal tetras, and a pair of mated rams. i bought a new guppy the other day, and acclimated him in the bag before putting him in the tank. ammonia 0, nitrate and nitrate 0 (i have a lot of plants) but this one guppy is just sitting at the top of the tank and on top of the filter. none of the other fish do so it is hard for me to see that he would be struggling for oxygen etc. he eats fine and doesn't have any visible illness as far as i am aware. tank temperature is 27c/80f. Any suggestions?