OK have you got actual results please? And can you tell me what and how much you're feeding?
15% weekly sounds a bit low, normally we recommend at least 25% a week with typical community fish - for big carnivores it may need to be more than that, but actual test results will help with that.
I do advise ditching the strips in favour of liquid tests which are more accurate and less expensive in the long run.
Other than that, the most obvious problem is your stock. Just the Oscar needs a 4ft x 2ft x 2ft tank on his own. Adding other large cichlids requires a much larger tank circa 8ft x 3ft x 2ft and of those you've added, only the Severum is a suitable tank mate. The others are all highly aggressive and not suited to a mixed tank of this size.
Please read these profiles and take note especially of tank sizes and behaviour/compatibility.
Oscar(
Astronotus ocellatus)
Green Terror (
Andinoacara rivulatus)
Jack Dempsey (
Rocio octofasciata)
Jaguar Cichlid (
Parachromis managuensis)
Texas Cichlid (
Herichthys cyanoguttatus)
Green Severum (
Heros efasciatus)
You will also see that these fish are not compatible in terms of water hardness and pH. You need a serious and fast rethink as this will only end with sick or dead fish.