First of all, according to various people, you may want to reconsider keeping your catfish and pond fish together. I posted a link below - or you can do a search yourself on the subject. In our pond in Upstate NY (similar in zone to you), I remove all my tropical plants in early October. We cut back our hardy water lilies which are planted directly in the pond and drop to the bottom the other hardy plants. Our pond is 3 feet deep and approx. 3,000 gallons so the fish overwinter in the pond. I don't know what you should do with a pond that small. We purchased a plastic 100 gallon tub at Country Max for our tropical plants, and I think that would work well for your fish if you decide to bring them indoors. You would have to add a pump or air stone for aeration and something for filtration. My husband altered the "plug" with a faucet and we can drain it pretty easily with a garden hose into our basement sewer come spring. Here is a link that might be useful: catfish in a pond

I'm no expert but I believe warm temps are key for these plants. Once it is warm enough they will start putting out new leaves and at that point I believe you can begin with the fertilizer. (That's when I start mine) If its too cold the plants don't do much. They just kind of sit there dormant. I don't think fertilizing in cool weather will do much.

Yeah, 1/4-1/3 strength fertilizer is generally a good idea. I usually wait until the 1st set of leaves are leafing out well (not counting the cotyledon/feeder leaves that emerge first and generally drop off) and the 2nd set is showing before I fertilize unless there's early visual signs of nutrient stress. Rarely do I have to fertilize before that point. It all happens rather quickly after the 1st set of true leaves emerge.

You can feed them any time after they get the second set of leaves. What you need to do now is stop watering them with the sprayer, you might be encouraging "damp off". Water from the bottom.