Usually, yes.

That is, assuming the following conditions are met:

• It’s freshwater (just to state the obvious…)
• It’s not too hard (prolonged watering with hard water will eventually saturate your plants’ soil in salts and dehydrate them. This will happen too with tap water if it comes from calcareous underground sources)
• It contains nitrogen.

The first 2 conditions are necessary, otherwise the water will actualy harm the plants in the short or long term.

The third condition is why we do water changes in the first place: to get rid of nitrogen in our tanks. So >99% tanks will have loads of nitrogen to export this way.

Some setups, however, can manage tank water with almost undetectable levels of nitrogenous compounds. It’s not the easiest thing to pull off, and it requires lots of plants in the tank and not too many animals in there. In the same way all those plants in the tank “eat” all the nitrogen, they will also exhaust all other nutrients from the water — which is great to avoid algal growth. That, however, removes any beneficial effects of watering plants with that tank’s water, that is, extra benefits when compared to “regular” water.

In my case, despite having a couple of tanks with nearly 0 nitrates, tank water is actually a lot better for my plants than tap water, because local tap water is extremely hard while my tanks’ water is very soft and has a rather low pH, which is what most ornamental plants like.

In any case, chances are it’s going to be better than tap water.