20 Gallon Tank Platy and rainbow shark issues

My tank has been set up for about 6 months (new tank) and the water statistics are fine (0 in ammonia, nitrate, etc. and 7.2 pH).The fish include two fancy male guppies, 2 cory cats, 2 dwarf gouramis, a red wag platy, and a rainbow shark, as well as two live plants. All fish except the shark are at their adult sizes (1-2 in. respectively), and the shark is about 2.5-3 inches long.
Anyway, when I first got my platy, he started by swimming down to the bottom of the tank, near my shark. For the next few days, he would actually nibble at my shark's fins (at the time, the shark was actually twice as big). After a week of this, the shark began to chase my platy away. Whenever my shark sees the platy, he will chase the platy back to the top and around the tank. After this last water change, he seems more peaceful, but at least once a week, the shark will chase the platy whenever he sees him.
They seem peaceful now, but I'm still worried about what caused this behavior, and if it could return. Honestly, I believed that platies were nice, but I'm still worried about my shark's health. (the shark would NEVER nibble on any fish. Though he would chase the platy, he never left a bite mark on any of the fish.) The problem is solved for now, but I don't want my platy to turn violent again.
What caused this behavior, and could it reurn? If so, what could I do to save Finn (my shark) without hurting my platy?

 

Hello Rainbowshark. A Red Tail Shark (RTS) is and can be very territorial. You may have seen information in places that the RTS can do well in tanks as small as 10 gallon which I have to disagree with. From personal experience I feel that a RTS or Rainbow Shark both, should actually be housed in 55 gallon tank or more with lots of hiding places. Chances are your RTS is going to be "King" of your tank meaning he thinks all of the territory is his. You may see a good bit more aggression from him. Remember too, he is by no means an adult at 2-3"s. I would highly recommend that you consider re-housing him into a larger home or possibly returning him for store credit.

We'd love to see photos of your tank and fish if you have the means.
Ken

 

Welcome to fishlore. Sharks are terratorial maybe this is why he chases your platy.

Hello Rainbowshark. A Red Tail Shark (RTS) is and can be very territorial. You may have seen information in places that the RTS can do well in tanks as small as 10 gallon which I have to disagree with. From personal experience I feel that a RTS or Rainbow Shark both, should actually be housed in 55 gallon tank or more with lots of hiding places. Chances are your RTS is going to be "King" of your tank meaning he thinks all of the territory is his. You may see a good bit more aggression from him. Remember too, he is by no means an adult at 2-3"s. I would highly recommend that you consider re-housing him into a larger home or possibly returning him for store credit.

We'd love to see photos of your tank and fish if you have the means.
Ken

Great info Ken. We posted at the same time but you got this one m8.
Nick

 

Thank you.

 

For those of you interested, I was able to get a picture of my shark and my tank onto yhe site. Please look, and I'll try to post more of my other fish.