No, ideally you should not feed FW food to SW fish, though it won't really harm them but it will be missing some of the essential additives (minerals, vitamins) and other ingredients that are more beneficial to the health of your SW fish.
In a pinch, it would be ok for limited feedings until you purchase the SW appropriate food.
Next - it would be extremely unusual for a SW fish to develop a fungus due to the high concentration of salt in the water of a SW tank.
Can you post a pic of what is going on with your fish so we may attempt to identify the issue.
Additionally, since we don't know what is wrong with your fish, obviously we cannot tell you if ingesting any of the water from the tank could harm you in any way. If you do not feel well, best to check with a medical professional.
Can we assume you didn't swallow the tank water?
As mentioned in an earlier post, either buy a pump to to your water changes (this is what is commonly done), or if you prefer to continue using the 'manual' method, DO submerge the hose, put a finger over each end, and then bring one end over the edge of the tank and remove your finger once you have it directed toward a vessel (to catch the water you are taking out) and allow the siphon to begin.