You can drain the other tank and refill it but don't wash the gravel or filter because you will get rid of any beneficial filter bacteria and start the cycling process over.
You can add a liquid bacterial supplement (available from most pet shops or online) and add a double dose every day for a week, then pour the remaining contents into the tank. try to add the supplement near the filter intake so it gets drawn into the filter where it belongs.
If you spread the fish out between the two tanks, reduce feeding and do big water changes, you should be able to keep the ammonia levels down until the filter cycles.
Out of curiosity, why don't you use tap water in the tanks?
You can add a liquid bacterial supplement (available from most pet shops or online) and add a double dose every day for a week, then pour the remaining contents into the tank. try to add the supplement near the filter intake so it gets drawn into the filter where it belongs.
If you spread the fish out between the two tanks, reduce feeding and do big water changes, you should be able to keep the ammonia levels down until the filter cycles.
Out of curiosity, why don't you use tap water in the tanks?