my sister and her family(the inlaws) have done no research and has had a fish tank with 4 fish in it for the longest time but this is the problem: the fish are as follows 2 oranda goldfish, 1 black moore goldfish and a pleco (common). i would take the fish but my tanks are full already. how big of a tank would they need if they wanted to keep all the fish?
Goldfish require a much higher volume of water per fish along with a lot more filtration for the tank they're in than do most other species of aquarium fish we normally keep. For 2 orandas and 1 black moor, I would recommend a
55-60 gallon tank at minimum ... plus
double (or more) the usual level of filtration than a tropical community tank setup would need.
More specifically... for slender-bodied, pond-type goldfish such as comets, commons, wakins, and shubunkins - plan on giving them 20 gallons per fish. For fancy varieties like orandas, moors, lionheads, and so on - plan for 15 gallons per fish. By following this basic 'rule of thumb,' we're able to maintain a goldfish tank easily by simply performing regular routine tank maintenance practices; i.e., weekly partial water changes, filter cleaning, gravel vacs.
Plecos should never be kept in the same tank with goldfish for several specific reasons...
Aside from (#1) their natural taste for the slime coat produced by high bodied fish, like goldfish, they also (#2) need vastly different tank setups.. tropical conditions vs. coldwater/temperate conditions.
(#3) Competition for food becomes a big problem, as well, for the much less gregarious pleco living in the same tank with such active, boisterous goldfish. Goldfish are omnivores who are constantly looking for leftover bits of food -- which is the same as that needed by the pleco. This makes it a constant challenge to keep a pleco well fed in the same tank.
Most importantly, goldfish are still a pond fish species despite the fact that we keep them in aquariums. They are much more prone to problems of parasitic infestation/illness because of this.... goldfish are also more susceptible to illnesses/disease resulting from their unique digestive limitations. Invariably, when the need for medication arises for the welfare and good health of the goldfish, (#4) plecos cannot tolerate the same medications and dosages needed for treatment.
Needing to keep the water clean and water parameters stable in goldfish tanks is also a major consideration, as (#5)
both goldfish and plecos produce an inordinately high level of waste. Adding a pleco to an already heavy bioload makes it that much more difficult to maintain the good water quality conditions necessary to prevent sickness, disease, parasites, and other common health problems for the goldfish.