Disease or bad luck?

[Eric]

I am hoping for advice, and so the long message.  A few fish have died in my son's tank in the last couple of weeks.  He is 6, so we have names for the fish and tears when they die (no funerals yet).

I am trying to sort out if there is a disease at work, and what I should do next. 

First, it was a swordtail we had for about 3 years.  She was too thin and arch-backed for long time.  I figured it was age.  I have had them go like that before, but not for a while. 

Then, a week later a betta, which looked just fine until the last day when he stopped swimming/eating.  We had him about a year.  I became suspicious.

Third, a platy, who was like the swordtail -- about 3, same appearance.  I was now hoping it was age again.

Yesterday a molly died.  We bought him a week ago just before the betta died (maybe a bad decision, but to cushion the blow).  The last day, one pectoral fin looked a bit gray, sticky.  Early fungus or velvet seemed possible to me, but I never thought he would go so quick.

No change in the routine.  Water is fine.  The other fish -- three guppies, two platies -- seem good, though one platy is also 3 or so and looks much like the departed swordtail and platy.

I am planning to wait a couple of weeks to see what happens.  Maybe I should do more -- but what?  I could medicate with something general, but what and it seems a bit late.  I thought about clearing the tank while the remaining fish look good, but my other tanks are fine and I am not keen on spreading something.  I don't have an empty tank running.

Thanks for any ideas.

Eric