sorry kissez but, in all fairness, i feel that i need to elaborate on plecos a bit. your post made all plecos sound like evil goldfish killers that cant wait for you to turn your back. of course, this is far from the truth.
plecos (common) have a
slight tendency to be somewhat aggresive (this is a relative statement though). moreso, the aggression is towards other plecos. they
do not actively persue a goldfish (or any healthy fish) to "suck on". however, if you have a sick fish (lethargic) in the same tank as one, you could possibly wind up with a munched on goldie. BUT, goldfish will also do the same exact thing to each other. if a pleco (common) is malnourished and/or in too small an environment, you could run into problems as well. this is not to say that all commons are perfect angels.
many, many, many fishkeepers have been keeping common plecos in a wide array of aquariums with not a single problem other than the pleco outgrowing his environment. i am one of them.
there are other types of plecos that have a much better reputation with goldfish. a couple to look out for are bristlenose plecos (bushynose) and rubbernose plecos (rubberlip). both stay pretty small and from what ive read, seem to remain vegetarian. both are excellent candidates for your 40 gallon and will keep it spotless of algae. (literally spotless, overnight) however, there is 1 type of algae that i personally know that they cant clean up, thats spot algae. but, spot algae is generally found in well established aquariums, grows VERY slowly and can very easily be maintained by a monthly glass scrubbing.
so, with this and other available information in mind, there is a general rule of thumb that basically says "novice aquarists shouldnt attempt mixing common plecos with
fancy goldfish". once youve got a better understanding of their needs (and individual dispositions), it can be attempted with successful results.
i highly suggest a rubberlip or bushynose for your tank (they are just plain awesome janitors). just ask anybody who has one if they are willing to part with it. ive got a feeling that the answer would be
if you need any info on them, just check out the compatible fish forum here at kokos.
hope this helps "clear some things up."
paul