baby yellow belly slider with soft shell

Question
QUESTION: hey i have a baby yellow belly slider that i have had for about 5 to 6 months. he is about the size of a golf ball, his shell is soft on the top and bottom , and on the end of it, but right above his tail on the edge of his shell is super soft and i dont know what to do! im really worried about him! so you think he is just not getting enough calcium and protein? or is it something else? and im used to algae growing on his shell and back legs, but today there was algae on his head and front legs and the soft skin between his legs and shell. note; he is not in filtered water, but is cleaned regularly: please help me!

ANSWER: This is a case of improper husbandry. You have got to correct the care with him or he will die.

You need a filter  system that will keep the water moving, stagnit water is unhealthy and causes him to breath and live in its own pee and poo. Frequent water changes does nothing for him really except stress him out,

Next, he needs uvb and heat lights. you make no mention of having either The lack of each will cause him to get shell rot. Which you are describing.
Next at 6 mo old he should be MUCH bigger.

So, the water has to be 3 times his height- if he was standing on his back legs, the water needs to be 3 times deeper than he is.

you need to get UVB on his ASAP this can include taking outdoors for a period of time. watch out for him so a bird or other preditors do not get a free meal.
and do not put putdoors on a glass tank as they do not release heat and he can die.

Im not sure what youre feeding him so i can not assit there but I can tell you that he should be alot bigger, and the shell may have shell rot.

the best advise i can give is find a local herp vet and let them give injections to help kick start his recovery. Or a rehab center for reptiles.

---------- FOLLOW-UP ----------

QUESTION: yes i do know i need to get him a new home and i will asap. thankyou i will take care of him right away.

ANSWER: if youd like i can help you either correct care or find a rescue that will assist

---------- FOLLOW-UP ----------

QUESTION: well how so? i would like that. but for the uvb light would i put it over his whole tank or just one certain area? also.. what is a putdoor? im new to this whole thing.

Answer
PUTDOOR was a typo I meant outdoors.

If youd like you can research google for care or I can make you care sheets.
UVB needs to be on one end with a heat light and this is referred to as a basking area.
water needs to be 3 times his height
so if he was standing on his back legs go 3 times deeper then on one end put a rock etc something he can get up on and be out of the water this is the basking area then the lights shall be over this area.

to make it a little easier how about telling me what do you have for him and ill assist from there.