Hi,
I'm in Zone 5 and have 2 half-barrel ponds, with three small goldfish in each. I've kept the filter running, as well as a bubble stone in each barrel over the winter. I also put small stock tank heaters in each to keep the water from freezing, which kept the water temp at about 40 degrees all winter. I fed the fish 1-2 times per week, which I now see I shouldn't have done.

I have a Tetra Whisper filter and am wondering if I should change the filter cartridge? I've rinsed the cartridge with pond water several times to clean it, but it is looking really grungy. But I don't want to remove the good bacteria in the existing cartridge if that's going to be an issue.

Also, what to do about the massive amount of string algae? I remove some weekly with a toilet brush, but as my ponds are so small, there's so much in there that my fish can hardly swim around. Or will it dissappear on it's own once my plants start growing again?

Thanks!