Please Help

I have a 90 gallon tank that I've had up and running for almost 3 months, Spent the first month with just guppy's in it to help cycle it in.

We'll we added a couple of pictus <?> catfish on Saturday afternoon and one of them went belly up by Sunday afternoon. I've been doing testing the water quality every 4-6 hours every time I add fish just to be sure that the ammonia levels don't go dangerous (Using Nutrafin testing kit it shows ammonia at .1 which is one level above none detected) the Nitrite level shows about the same level. Water temp. is a constant 78 degree's and the ph is level at 7.2.
The problem that I'm having now is that most of the tank mates not all are going from the bottom to the top and then back down to the bottom. I changed out about 20% of the water in the tank thinking maybe the ammonia levels were starting to rise but they haven't moved. Should I do another water change?
In the tank we have:
1- snow flake moray eel
4- clown loaches
5- Elephant noses
1- Pictus Catfish
1- Pecostimas <?>
3- Parrot fish


and a few guppies still in the tank that began as feeder fish about 2 1/2 months ago.

Thanks in advance for any suggestions

1- Using Nutrafin testing kits for ammonia, Nitrites and PH levels. Also we are using a testing strip that while not as basing my results on it, I'm using it to make sure my other readings are not off base or that one's not showing things are fine and the other is not.
2- We set up the tank before Christmas added feeder fish after about 48 hours and didn't add any of the other fish to this tank for 6 weeks while adding adding cycle to the tank as directed.
3- As per my last testing at 9 am:
Nitrates show between 20-40 ppm mg/l
Nitrites show between 0 and .1
ammmonia shows between 0 and .1
PH 7.2
water soft
Total Alkalinity between 50 - 50 ppm

I took a water sample to 2 different LFS and they tested it in using 5 of the Nutrafin test kits and they all say they don't think it's not a water quality issue? We had just added the 2 pictus cats and 3 clown loaches to the tank in the past week, could it be stress? That is causing the behavior of going to the top of the tank by a couple of the fish?