Dear Sir/Madam, 

I am writing to complain about the a [1] problem in the supermarket. I am a housewife. Yesterday I was shopping in the supermarket and I was considering what food I should cook for dinner. When I walked by the seafood section, I fell off over [2] on the ground. There was some water on the floor. I got [3] hurt because of that.
When I got up, I understood what was happening had happened. Due to the alive live [4] fish, they make splash the water out of the cave container and it stays on the floor. I would like to say that you should make some changes so similar accidents could can be prevented. The supermarket should keep the floor dry and clean, also and should not let have the cave container full of water. Just half of it full of water will be OK.
That is all I want to express. Thanks for your reading and if you use my suggestions, I would be grateful for that [5].
Yours, 
Mary

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[1] You have not yet described the problem.  Before you describe the problem, you must refer to 'a' general problem.  After you have described the problem, you can refer to 'the' specific problem.

[2] The expression "fell off" means that you were on something.  For example, "I fell off a horse."  If you were not on something, we say, "I fell over."

[3] When writing an IELTS essay, try to avoid the word 'got'.  There is usually a better word.  Here you could have written, "I was hurt because of that."

[4] The word 'alive' is correct.  However, we tend to use the word 'live' if it procedes the name of the animal.  For example:

"The fish were alive." (Here, the word 'alive' follows the noun.)

"The live fish." (Here, the word 'live' precedes the noun.) 

[5] A more natural expression would be to write, "I would be very grateful."

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Great job!  You addressed all the points of the task.