Plecostomos? Most of them are tropical fish that need heated tanks, a few species manage in cold water, but you would have to find that exact species and hope that it is not mislabled like many plecos. Even if you found one of those species, the pleco has likely lived in tropical conditions it's entire life. you cant take a fish out of 78 degree water and put him in 70 degree water right away, you should never change the temperature more than 2 degrees every 2-3 days. or the fish can go into shock. And the changes in temperature that occur from day to night take a lot of getting used to.
Also, plecos are dirty and bulky fish, even dwarf species need 30 gallons minimum, they have to live in the dirtiest part of the tank. 4 goldfish need a minimum of 55 gallons with an extra strong filter, the ammonia alone could have been too much for the plecoe in that tank over time.
Presumably you mean a plecostomus. They are tropical fish, and likely died of shock.
1. Not all plecos eat all kinds of algae. Their diet should be supplemented with algae wafers, catfish wafers, and shrimp pellets as well as fresh vegetables to provide a nutritious and more complete diet.
2. Plecos should not be kept with goldfish. Plecos are tropical, goldfish are considered coldwater fish. Also, plecos and other types of algae eaters may attack goldfish and suck on their slimecoat.
3. Adding another fish to an already overstocked aquarium is a bad idea. Plecos, like goldfish, produce a lot of waste. Your 20 gallon doesn't need anymore stress on it's already overwhelming bioload.
4. Goldfish are best kept with only other goldfish of the same variety. Plus, goldfish are omnivores, so they eat algae.
If you have an overabundance of algae in the tank it's not a sign that you need an algae eater, it means you need to limit light exposure to the tank and/or the water quality is poor.
With goldfish, the only option you have for an "algae eater" are mystery snails. They actually do a very good job.