thank you to everyone who has posted some help. i tested the water with some test strips by Jungle Labs. it reads:
nitrate: 0-20 ppm
nitrite: 1.0-3.0 ppm
hardness: 75-150 ppm
total alkalinity: 180 ppm
pH: 7.2

the tank is in a fairly climate controlled classroom so i don't think temperature change would be the culprit.
they're behavior is pretty stressed. some are suspending toward the top, some are awkwardly & slowly swimming. when they were healthy, they were laying at the bottom.
the filter is made by tetra and was built for koi ponds. it is rated for up to 1000 gallons i believe.
they are channell cats.
i plan to replace about 100 gallons of water today. it won't be declorinated, but i have some bait-saver additive that has worked when i had to add water do to evaporation.