Are you saying that you've kept your rhom for 31 years!!!! Please tell more about this, pics if possible please...

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Yes it is 30 years plus in my tanks. I have always kept it by itself and move it about every two years between 180g and 120 g tanks when I do a teardown on the tank it is in.
Here is a pic taken a few days ago and it is in a 120g.
I saw my first live S. rhombeus in 1957 and again in 1971. I am grateful to Frank Glennon of the Steinhardt Aquarium San Francisco, California for informing OPEFE that the oldest known (to me) piranha in captivity is *still alive (See image at left of the actual fish). I personally saw this beast in September of 1971 when it was only 6 inches TL (= total length). After 28 years in captivity this fish was only eleven inches TL (March, 1999) when it died. It was reported to me by a hobbyist the S. rhombeus discussed above is no longer alive (Sunday, August 6, 2000). It was thought the fish jumped out of the aquarium or died some other way. Reportedly, the fish died sometime in the latter half of 1999. What a sad end to fish that lived for over 28 years.
I feed mine once or twice a week. Mackerel, Herring, Smelt and Shrimp. I keep a few guppies in the tank and several large mystery snails to work on anything it does eat.
It moves every two or three between 180g and 120 g tanks when I do a massive clean out. Undergravel filters with a couple of airstones. A couple of bogwood with plastic plants attached.
The fish is in it's 31st year in my tanks. It is only about 10" to 11".