Kurtosis

Unfortunately he didn't make it, inspected the gills but didn't see any signs of damage or parasites ( I don't have a microscope though) If anyone has any ideas what may have caused this so I can prevent it in the future it would be appreciated. Community tank is a 240 litre ( 48 inchs by 18 inches ) I believe this is a 65 gallon ? 2 inch sand substrate, Large piece of bogwood, black lava rock and various plants. Running 2 sponge filter ( for aeration and so I have cycled filters for rainy days ) And a ultramax Cannister filter running sponge and bio rings.

There's angelfish in the tank but they only seem to bother each other, will monitor and update if situation changes.

Male kribensis in community tank was laying on bottom behind cave and appeared to be panting. Other fish no signs of stress. Any help identifying please ? 26/05/22 Ammonia 0 Nitrates 40 Nitrites 0 Gh 8 Kh 6 Ph 6.5-7 temp 26°c 79°f Male krib in QT with 1.5 tbls salt / 2 gallons