Have some health problems was thinking about a plant based eating lifestyle + fish only, ppl say that's a bad idea it can cause more problems. True? Ty

Food plan. Please start by seeing your doctor so that you both know what you are dealing with. If you think you have some nutritional imbalances consider getting a Micronutrient Test like one from Spectracell.com. Then you will know what you need to eat or supplement with. A plant based diet w/fish is a possibility but you need to be more informed before doing that. Peace and good health.
Generally healthy. There is good evidence that a predominantly plant-based diet is healthy but I believe most people benefit from some animal-derived foods and fish is a relatively good source of protein, essential fatty acids, B12 etc. though I advise wild fish with low mercury.
There is no one diet good for everyone but this would be excellent for most. See https://drrandybaker.com/2013/01/20/diet-and-health/
Not. The fish is adequate protein and will get enough protein and Vitamin B12. Though for specific diet your nutritionist or you Bariatrician will help.