Do I still have to cycle the water in a new tank if I use 50% water from another existing tank?

[freshwaterparadise]

And the other 50% treated tap water

 

[FreshyFresh]

Using old tank water is a myth.

The beneficial bacteria you need to process ammonia lives on all the surfaces in the tank, not in the water column.

 

[Rbishop]

You will need a good liquid test kit to monitor and display the cycle of your tank. Just old tank water will not cut it.

 

[fishorama]

You can use some filter media from your existing healthy tank in the new filter almost half. &/or you can put some old substrate in a mesh bag or piece of panty hose in the new tank in the flow of the new filter or in the filter. If the old substrate is the same as the new you can also add it on top. I often use cycled filter sponges or ceramic media to help start a new tank.

If you're not in a super hurry you can run the new filter on the old tank for a few weeks to seed it.

But all these methods will only allow you to add ~half of the fish load of the old tank at first. I'm talking about body mass not number of fish unless they're similar in size.

& as Bob said test often & be ready to do large water changes as needed.