I live in Australia and i found out that many products i buy (like frozen vegetables, hummos, vegetarian soups etc) have this label:
This product may contain fish or nuts or ..etc.

After searching for the meaning of (may contain).. this is what i found:

"The statement ‘May contain traces of …" is put onto food labels when manufacturers believe that the food is at risk of contamination from a problem ingredient such as peanuts or fish (called an allergen). This usually arises when nut-free biscuits, say, are baked on the same line as biscuits that have peanuts. Despite their best efforts to clean the production line, you can never rule out the chance that a small piece of peanut from one batch may accidentally get into another batch or dough".

So does that mean that eating such food is halal since fish is not really one of its ingredients?

Yes it is halal and you are not required to investigate beyond the apparent labelling according to all maraji of taqleed