The fish I have a tetras (neon, flame, albino glowlight, yellow
phantom, rosy), silver shark, king tiger plec, moonlight gouramis and
of course clown loaches. The shark, loaches and plec have not shown
any signs of disease.

What additional filter are you using? The Juwel filter is rated for the tank
with a community of small fish, not a highly stocked tank with larger fish.