Firstly, even small plecos need a tank that is at least 30 gallons. The plecos that I am assuming you bought are common plecos that need at least a 75 gallon tank because they can grow to 2 feet long.
Second, Stocking guidelines for gold fish are a 30 gallon tank for 1 and then an additional 10 gallons for each gold fish after that.
Both of these fish are HUGE waste producers which will put out tons of ammonia. This is probably the reason your plecos are dying.
Have you ever tested the ammonia, nitrite, and nitrate of your water? I am worried that your filter can't keep up with the ammonia production in your tank. Skip the test strips and instead buy an API master kit.
Your tank cleaning is excessive. For a properly stocked tank a weekly 50% water change is the suggested cleaning routine.
Take a look at this link, it will have a lot of information on caring for an aquarium.
Guide to Starting a Freshwater Aquarium - Aquarium Advice
Lastly, this was posted in the saltwater section of the forums. You might get more replies if you posted in the fresh water section