Those red leaves..

They are very shiney to is that where new fruit will come.

I love a mango tree flushing growth! Some avocado trees have the same color.

My other Mango seedling..hardly grew this year. It has even redder new growth. Maybe the soil is too heavy? Or its taking time to root? Its already 3+ years old and only 3' tall. I'm worried about that one.

Does your mango seedling receive several hours of sun? I know when mango trees have too little light they decline and eventually can die.

Wow! 3 years and only 3' than not too tall. My seedling mango is 1 year old and it's almost 3' tall. I think it would be taller if it had been planted in the ground. I'm thinking about it.

Maybe your mango just needs a little fertilizer.

Have you posted a pics of your smaller mango tree?

I can post new pics. Its strange...the leaves it did grow...are deformed.
The wife said for me to move it. I might but this isnt the best time.

I just read that it can be from overwatering lowering oxygen levels at the roots. Maybe I did.

Hopefully, it's not anthracnose! That can sometimes cause the leaves to look deformed.