There are many problems with keeping goldfish and turtles together, and it is not recommended to cohabitate them. Here are the main reasons:

Conflict in Living Habits
Goldfish require a deep water environment, while turtles usually adapt to a shallow water or semi - aquatic environment. If a turtle stays in deep water for a long time, it may not be able to breathe properly and may even drown.
Turtles need land space for sunbathing and resting, whereas goldfish rely entirely on the aquatic environment. It is difficult to reconcile the different needs of the two.
Predation Risk
Turtles are omnivorous animals, and their diet includes small fish and shrimp. Goldfish are small in size and are easily preyed upon by turtles. Especially when they are hungry, turtles will actively attack goldfish.
Water Quality Pollution
Turtles excrete a relatively large amount of waste, which can easily contaminate the water quality and affect the health of goldfish. Goldfish have relatively high requirements for water quality. Deterioration of the water quality may cause them to get sick or even die.
Disease Transmission
Turtles and goldfish may carry different bacteria or parasites. Cohabitating them can easily lead to cross - infection, increasing the risk of diseases.
If you insist on cohabitating them, the following points should be noted:
Select Suitable Varieties: For example, Chinese pond turtles or Chinese striped neck turtles. These varieties have a gentle temperament and are less aggressive.
Provide Sufficient Space: Use a large fish tank and divide it into a deep water area and a shallow water area to meet the different needs of goldfish and turtles.
Ensure Sufficient Food: Feed the turtles first until they are full, and then feed the goldfish to reduce the possibility of turtles preying on goldfish.
Change the Water Regularly: Keep the water clean to avoid pollution and the spread of diseases.
In conclusion, separate rearing is a safer and more reasonable choice. If you want to enjoy watching both of them, you can consider using a specialized fish - turtle cohabitation tank, but you need to strictly follow the above precautions.