Hmmm They get to 5" and 3ft is 36" i think youll be ok
here some info on tehm
Scientific Name: Metynnis argenteus
Common Names: Silver Dollar
Family: Serrasalmidae
Sub-Family: Myleinae
Origin:Guyana, Amazon east of the Rio Negro
Sex: Anal fin of male is larger with a reddish tinge
Breeding: Egglayer
Size: 5 inches
Level: Middle
Temperature: 75 - 82 F
pH: pH 5.0 - 7.0
Hardness: Soft to medium hard water
Tankmates: Shy, peaceful, community fish
Diet: Herbivore
A longtime favorite for community tanks, there is little doubt where this attractive silvery fish got it name. Although Metynnis argenteus is the most commonly encountered of the sub-family of Myleinae, keep your eyes open and you'll no doubt see several other varieties at your local pet shops. I enjoy visiting shops in other towns to see which varieties they have on hand. Look for spotted or even striped specimens, as well as ones with fins of different sizes and shapes. These fish are Charcins, and are sometimes mistaken for Pacus or Piranhas.
Almost every account I've read about Silver Dollars describes how shy they are, in fact manuals often include cautionary remarks about being sure the other fish don't eat the food before the Silver Dollars get to it. However, I've had many Silver Dollars and none of them had any trouble competing for food. They are definitely skittish, and will leap out of the tank if the lid is open and they are startled. I've found that over time they become rather tame, and can even be tame enough to hand feed. One of my favorite Silver dollars even allowed me pet him regularly. In my experience the ones who became tame were single specimens, rather than those kept in schools. Speaking of schools, it is advisable to keep Silver Dollars in small schools of four to five fish whenever possible.
Favorite Foods
Listed as herbivores, Silver Dollars must have skipped school the day they were teaching what a vegetarian is supposed to eat. I've fed my Silver Dollars beef heart, bloodworms, and brine shrimp, in addition to fresh vegetables such as peas and lettuce. Once I read that they will eat breakfast cereal, so I tossed a few flakes in the tank. Sure enough, the Silver Dollar snapped it right up (as did several other fish). Although they'll eat a wider diet than the average herbivore, they still need to eat more frequently than carnivores due to differences in their digestive system. Several feedings a day are recommended to keep them in top condition. A well cared for Silver Dollar will easily reach 5 inch size, or even a bit larger.
Habitat/Breeding
The ideal habitat for Silver Dollars provides them with a large area to swim, as well as places to hide. Subdued lighting and dark gravel is recommended, as well as plenty of plants. When it comes to plants, the Silver Dollar remembers that it's a vegetarian, so consider using the plastic variety. Otherwise be prepared for some casualties among your plant population. Water conditions are not critical, unless you are interested in breeding them. To breed Silver Dollars the best tip I can give is to use soft water. Although the parents will not eat the eggs other fish will, so they should not be bred in a community tank. After several unsuccessful attempts I finally had a pair spawn when I set up a tank with very soft water at 80 degrees. The eggs drop to the bottom where they hatch 3 days later.
For anyone who has at least a 30 gallon or larger community tank, I'd recommend trying out a school of Silver Dollars. You'll find them an easy to care for, long lived, and peaceful addition to your tank.
Gl DiscusAngel
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