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Some barking sloughing off is normal in trees. There can be more serious causes.

Examine the tree for signs of pests, as they can attack the bark of the tree.
Drought can cause stress to the tree.
Poorly draining soil can cause Cotton Root Rot.
Crown Gall can also attack the tree.

https://www.gardeningknowhow.com/edible/nut-trees/pecan/growing-pecan-trees.htm

I didn't find Pecans listed as toxic to fish, but I would check with an expert in fish. This is outside of my expertise.

This sounds like what's called producing "pops". This questions has been answered at a university, and the link is here: http://aces.nmsu.edu/ces/yard/1998/083198.html

This depends on the type of fruit trees. Some trees will have problems with being near pecans, as the roots can interfere, both physically, and chemically with your other trees. Some roots can extend out to 100 feet or more. Sometimes it can be safe to plant within 50 or 60 feet, but this will depend on the trees you intend to plant.

This article will give you more information: https://www.gardeningknowhow.com/edible/nut-trees/pecan/pecan-tree-toxicity.htm

As long as it is a naturally grown tree and not a graft. If this was a grafted tree then it will not produce the same fruit, or even any. But Yes, you can grow a tree from a sucker. This can be easy to do if you dig down and make sure to get some root growth. This will make things easier.

Here is a collection of articles for more information on the care of the tree: https://www.gardeningknowhow.com/edible/nut-trees/pecan/

This can be difficult to control. If you were to put something on them, they would find a different area to grow out of. The only good way is to keep them pruned. Usually these suckers are a response to root damage, or other damage within the tree. The best way is to remove them. You can also use round up to pain on the site once cut, but you must be very careful, and only use a very weak solution. This can help some, but they will reappear on occasion. It is best to find out what problem, if any, the tree has.

Can you include a photo? Seeing will help me to determine the best method of action.