Strange Silver Dollars?

[finchfarm]

I have a 75 gallon tank with three Silver Dollars, though technically one is a Red Hook Silver Dollar. The Red Hook is eight years old and the other two true Silver Dollars are about half a year old.

When I first got the true Silver Dollars they were pretty tiny and did nothing but hide behind one of the plastic plants in the tank. When they got bigger they came out of the plant and were a lot more sociable with the other fish and didn't act as scared. They're about three inches long now and I've noticed they've resorted to going back and spending most of their time in the plant hiding. I know it is natural for Dollars to be shy, timid and fearful fish, but I wasn't expecting such a dramatic turnaround from a pair of fish that was out all the time and now do nothing but stay hidden in a plant. There are no new fish nor no new objects in the tank.

Also I've noticed that the bigger of the two Dollars sometimes develops a black spot on both sides of his body. It is on his body (not is head) but is roughly lined up at eye-level. The first time I saw this it scared me pretty bad because I didn't know what was up with a black spot being on my fish. However, after watching him the black spot faded and within a minute it was gone and he was all silver again. I just went and turned the tank light on this evening and found the same Silver Dollar (I know its the same because its the bigger of the two and it has a little chunk missing out of his tail that never grew back), had the black spots on it again.

Should I be worried about the disappearing black spot and the change in behavior. Other than that they are eating and acting perfectly normal.

 

[Tokis-Phoenix]

Silver dollars do best when kept in larger groups, it makes them feel more confident and less timid when this if the case, i would advise getting another 1-3 silver dollars and 2-4 more red hooks, but it depends on what other fish you have in the tank and what the exact measurements of the tank are.
Silver dollars are herbivores and will eat plants but ironically they also enjoy having planting in their tanks, it would be best to put a decent amount of fake plants in the tank and make sure you are feeding the dollars a diet high in veg like chopped up lettuce or blanch spinach or cooked de-shelled pea's etc to make sure the fish are in good health

 

[CFC]

The answer is numbers, never keep less than 6 silver dollars and prefferably more, they will school with other fish of the family so you dont need to add to the numbers of both species, although an odd red hook in a shoal of SD would look a little strange.

 

[finchfarm]

Thanks for you input everyone.

As I said, my tank is 75 gallons. It is about 1.5 feet wide, 1.5 feet tall and about 3.5 feet long. Theoretically, how many Silver Dollars would I be able to put in there without it being overstocked? I was actually considering going species specific for a while as I really love my Dollars.

Any ideas on the black spots? I checked them this morning when I went to get tank measurements and both of them were out, both of them had the black spots, though they were much darker on the one with the chunk taken out of his tail then the other one. The black spots are prescent on the fishes body, just behind the gill area and is in line with the eye. There is also a smaller one down towards they're pectoral fins. The dorsal fin on the one also appeared to have black rimming. The black spots/tinge is not permanent and goes away within a few minutes, usually. They appear happy, healthy and are eating well. The spots are prevalent on both sides of the fishes' body in the exact same spots on both sides.