Damn, that's cold. Where do you live? Alaska?
eek I think the best temperature for goldfish would be between 63 Fahrenheit and 70 Fahrenheit. If the water is to cold, this may be why the other fish have been dying. But do check the water temperature before putting a heater, and have some one check the temperature daily to see if nothing goes wrong.
Also when bringing a new fish into the aquarium, make sure that even after you quarantine it you don't 'toss' it in the aquarium directly. Put it in a plastic bag and leave that drifting in the aquarium for about an hour, occasionally adding water from the aquarium to it.
The plants I have currently are;
Aponogeton Ulvaceus, 69-74 F, I have yet to see the golds try to eat this one.
Hygrophila spp. 59-77 F, they love rubbing their bodies against it and occasionally eat a leaf, but since the leaves are big it doesn't do much damage.
Sagittaria spp. 59-77 F, this one tends to get a fine layer of dark green algea which golds often love to graze on. If the algea layer gets to thick it can suffocate the plant though, so it might require some cleaning. Just rinsing under running water or even softly rubbing the leaves will do.
I also recommend:
Vallineria, 59-72 F, looks a lot like the Sagittaria but it's leaves are thinner. A very hardy plant that tends not to get eaten, The male plant has cute white little white flowers that drift to the surface.
Potamogeton Densus, 50-59 F, I've never kept this one myself, but since it requires a low temperature I thought it might be good for your aquarium. It seems to be used mostly oxygenate and purify cold water aquariums.
Water Milfoil (Myriophyllum), 50-72 F, it has a tendency to end up as fish food, but that does depend on the individual fish. It's very decorative, but do ask the sales clerck fro extra information on this one, or look up it's latin name, because different "Myriophyllum's" have different water demands.
Water Moss (Fontinalis Antipyretica), 50-59 F. Locally we know this species as 'Wetterpest'. Which means as much as 'water disease'. It tends to grow quite rapidly and uncontrollably, so monthly weeding may be needed. Some fish love nibble-ing on it however, be it their leaves or the algea attracted to it, which is why it sometimes disappears quickly, leaving only ghastly bare stalks.
Eloda Densa, 60-68 F. Same discription as the Water Moss, just for a higher temperature.
Ceratopteris Thalictroides, 64-86 F. One word; "fish food". A lovely plant to keep, and apparently, in my experience, more often then not, to eat. I some times buy it just for that purpose; so the fish can have something to nibble on for a while.
Now bear in mind that not all golds will eat plants, but they do have a reputation for it. Some golds that never ate plants before may suddenly start with a new plant, or even an old one. Some golds may also suddenly stop.
In my experience the larger and firmer the leaves the less likely to be eaten they are. The smaller and softer the leaves are the more likely they are to become food.
A few other links:
http://www.aquarticles.com/articles/ponds/Greene_Plants in the Goldfish Tank.htmlhttp://thegab.org/Articles/GoldfishPlantsLowTech.htmlhttp://www.geocities.com/Tokyo/4468/gf_plants.htmlOften the best way to protect rooted plants is to eihter pot them in pots, baskets or with fish-safe soil and cover those with gravel and just hope your fish are not to enthusiastic.
Some plants do require mild "airiation" and their roots should not be covered. I'd put a mesh net to protect their roots around them and tie that with an elastic around the stem.
Make sure it's tied securely but not too tight so the plant can still grow and it's saps flow won't be obstructed.
Hope this helps.