Clown Loaches flashing

Hi all I need some advise. Today my Clown Loaches and a few others began
flashing like crazy. I have had them for a year and they have been very
healthy once I got them past the ich. I checked the water, NH3/NO2 is at
zero pH is stable at 7.6 and temperature is also stable at 75. I do ~ 40%
water changes weekly with gravel vac and filter cleaning. I took a few
slime coat swabs and checked them under the microscope No signs of parasites
of any kind were detected. I have added salt at about 1 teaspoon per 5
gallons incase it is fungus or some other sort of parasite I missed. Anyhow
I am open to suggestions?

Tank 55 gal 1 year old
filter 1 regent 40-60 1 penguin 330 biowheel
2 airstones
1 300 watt ML Neptune heater

population:

4 clown loaches
3 dojo loaches
6 platies\
3 sword tails
6 mollies
3 medium Opaline gouramis
3 small pearl gourmis
1 dwarf gourami
8 black skirt tetras
1 Pl@co