A common pleco can get as big as 10-12 inches. They also grow very fast, in a 30 gallon tank he will still grow until he is too big for that tank. I would say in a year he will be his adult size but long before then he needs a bigger tank.
I see in your signature you want an oscar, you know they need big tanks also
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Large fish not only need room to lay but room to swim and turn around. Pleco are also huge waste producers and get more so as they grow. In a tank that size it will really be hard to keep the Nitrates and ammonia down due to that. They also tend to knock over decorations and uproot plants just moving around in the tank.
Now don't get me wrong I love plecos but just want you to understand they need lots of room.
Lila I'm not sure what kind of pleco you have but commons usually grow very fast.
Yup ... ONE oscar needs at least a 50 gallon tank. And if you want an adult pleco together with that oscar ... I'd say you should get at least a 100 gallon tank ... (that would be best for your fish if they are to live good lives). So maybe you should resign from the pleco and the oscar and, instead, get some smaller fish that are compatible with a 30 gallon tank that you want? Also, don't think angels mix well with gouramis. Anybody? Do they?