Watery starter

Hello! I'm new to all this and am hoping someone can help me.

I was given some 100 year old sourdough starter and when it arrived I fed it and put it in the fridge (right away). I fed it equal amounts flour/water. It was watery going in. It had liquid on top as the rest settled. I hadnt touched it in a few weeks so i fed it again yesterday but added more flour since i thought it was too runny. I let it sit out overnight and there are bubbles, but still very watery. It looked like it may have risen maybe an inch or two in the jar. How can I fix this? I dont want to lose it! TIA