Common plecos can be kept at sub-tropical temps (i.e. temps 19-23 degree's C warm) which are the same sort of temps that fancy goldfish can also be kept in, however although a lot of people are able to keep common pleco's and fancy goldfish together successfully, common pleco's have been known to try and latch onto goldfish and suck their protective slime coats off.
I keep one of my common pleco's with my goldfish, its been in the tank for over a year and they are absolutely fine together, however i did take a bit of gamble mixing the type types of fish together- i did it because the pleco was constantly bullying my other common and sailfin plecos, i could only have 3 125gallon tanks though.
When i put the pleco in a temporary 30gallon tank while i was looking to buy the 2 other large tanks, the sailfin then turned on the smaller common pleco when the larger bully one was removed. So in the end, the only way i could keep them was to have them each in their own indvidual tank. However, only two of the larger tanks were fully/truely tropical, the 3rd one was my goldfish tank.
So i moved the large common pleco in with the goldfish, it was a little bit of a risk, but the two types of fish lived happily together.
The only time the common pleco doesn't get along well with the goldfish is when they steal his algae wafers and he chases them off- this is not a problem though as long as there is either a great deal of algae wafers or the goldfish are distracted with their own food, or they simply don't notice the pleco's algae wafers because the tank lights are turned off etc. So basically as long as i take care (which i always do) in feeding the goldfish and the pleco, they coexist happily together in the same tank and completely ignore each other.
So feeding the two fish is definately something you should take into consideration.
Common plecos and goldfish are big waste producers, and both fish grow large- one other problem that can arise with keeping common pleco's and goldfish together is that they do create a lot of poop, and if the tank isn't very over-filtered and the substrate kept spotlessly clean, the pleco may suffer as its gonna have to live on the bottom of the tank where the waste and ammonia is very concentrated.
In my goldfish tank, i have twice as much filtration than would be nesarsary for a normal fish tank, i also do a large 40-60% water change once a week and clean out every filter in the tank (there are 4 filters in the tank in total) once a week too when i do my once weekly water change. I have a fine white silica sand substrate in the tank that is very shallow, because of its shallowness and the goldfishes rumaging around in the substrate looking for food and the powerful filtration in the tank etc, the substrate keeps itself more or less clean, however i will occasionally net out goldfish or pleco poop that isn't shifting itself along into the filters.
Goldfish tanks don't need to be heated as long as the tank stays at a stable temperature all year round, however they do benefet in being in a cycled tank- if you already have a cycled/established tank, you can clone the filter media to instantly cycle the goldfish tank.
Goldfish tanks are high-maintanence tanks regardless of whether you are keeping fancy or non-fancy goldfish, and become even more high-maintanence when you add a big waste producing pleco to the mix. However, with the money, work and dedication such mixes can work out well
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The Common pleco is the only type i would advise for a goldfish tank since it is the only pleco i know of that can happily be kept at sub-tropical (but not coldwater) temps in the long term, as far as i am aware most other plecos are generally too tropical-temp loving to be kept in tropical temps. Goldfish can be kept in tropical temps, but it is not that ideal, and if they are kept at such temps the tank needs to be kept very well oxygenated because oxygen disolves less well in water the warmer the water is.
Edit: With the algae eating/cleaning abilities of common plecos, this is a bit of a double-edged sword. Only when common plecos are juvenile fish that they take a very keen interest in eating algae, but as they grow up and mature, they stop paying so much interest in the stuff.
I don't really get any algae in my goldfish tank, the glass gets a little bit slimey if i don't clean it for a month, but there's never any real algae. Occasionally at night i will see my common pleco cleaning the sides of the tank, but he doesn't do it half as much as he used to when he was a juvenile.
When i say common plecos and algae are like a double-edged sword though, common pleco's can actually contribute to algae problems if the algae came about in the tank because there was an excess of nutrients in the tank, because these pleco's poop so much their poop can make an ideal fertiliser for algae if their poop is not properly dealt with.
So overal, they are not the best algae eating fish you can get. Oto's make much more effective algae eaters, their diet is a lot more herbivorous than common pleco's and oto's are low waste producing fish too and can be kept in large herds/groups to effectively clean even the most algae covered tank in days or weeks. Unfortunately though oto's would probably be eaten by goldfish though (goldfish eat whatever fish they can catch which are large enough to fit in their mouths), soo....
