Ick in two tanks, only one heater?

Hi... Well, long story here. I'll shorten it up, though.

Bought 2 mollies, 2 pictus catfish.

3 (both pictus and one molly) came down with ick and died. I had tried to treat it, but I caught it too late and the heat didn't work at all.

Just before she died, one molly had babies. She died soon after. About a week or so later, I turned the heat down. (It had been at 88.)

I'd had no problems. But, I had noticed my black female that didn't get it was hanging around the filter. The tank was unlighted and I didn't see anything off.

Today, I bought 4 tiger barbs for one of my other tanks and filled up my last 10 gallon for a quarantine tank and moved the heater that had been with the mollies into the tiger's tank, hoping to catch the ick that I knew they had because fish from the store had it before they got sick.

I finally managed to find a plug for the light, and, surprise, surprise, my black molly has ick...

Now what? I only have one filter small enough for a 10 gallon tank. Do I risk putting the semi-aggressive tiger barbs with my mollies to treat them all? Or, do I attempt to treat the molly tank first? I know they have ick in the tiger tank for sure because I put decorations from the molly tank into the tiger's tank. The current created by my too big filter is a little much for the fish if I don't put something in there to break it up...

None of the barbs are very large.