Dam, sorry to here of the loss. I would say the fish have been poisoned, ammonia or similar. Not to much you can do except keep the water clean & see if the ones that have survived continue to do so. The water changes are a good idea, maybe get your water tested for peace of mind too.
With the filter, rinse out in your tank water too. No point killing any bacteria that may be alive.
As for me I am lucky, mum is well trained, & the gf dosen't give a s..t about fish, so if anything they get fed on the light side. No one will care for your fish like you do, unfortunately.
A tip, maybe next time buy an automatic feeder, yes it is dearer, but then you won't have to go through this rubbish again.
Frenchy