Pygmy corydoras dying off

[smm333]

I had a group of 6 pygmy corydoras that I purchased together about 6 months ago. They have always been healthy, eat well, school together, etc. A couple weeks ago 2 died, followed by a 3rd several days later. They didn't act like anything was wrong, water parameters were stable, heater is fine, and other tank occupants are fine. I did a water change just to be on the safe side. Today another died. Parameters,  heat, all fine. Other occupants are doing fine. Maybe they are just older fish? I'm so sad, because I just love these guys.

I will pick up more pygmies to bring back a school next weekend, but just wondered if anyone has any ideas why this happened and if there is something I should check that I'm missing? This is my 1st time with pygmy corydoras,  but I have other types of corydoras. 

These ones are in a 20 gallon planted tank with sand substrate, 76°, hob filter with sponge over the intake. They live  with a mystery snail,  2 (temporary) baby mystery snails, 2 male endlers, and a female honey gourami. 

[lefty o]

if things test well, and other fish are fine it is very possible they are just old.