How large is your tank and are there any live plants?
The nitrite reading indicates that you have messed with the cycle capacity and this should be, as noted, zero. Replacing the filter does not remove all of the bio-film (ammonia and nitrite oxidizers), depending on the tank/filter it may not even remove half of them but that will still upset the balance.
I'll assume that you are using city water supply and your are treating the water with some sort of conditioner... if it's Prime, then the nitrite will be rendered non-toxic for a day or two. I'd suggest not only doing the 50% change, but change some every day or two treating it again until the nitrite subsides... you don't mention ammonia, it will probably be high as well but Prime handles this... some conditioners don't do everything.
I was going to suggest not wasting your money on more biomax as it appeared, at first glance, to be the typical ceramic material as others that are just too fine porosity to be of use in the aquarium setting. So before I put foot in mouth I thought I would check it first.
I have biomax in the office aquarium filter so I pulled one cylinder out, it's not hollow like some are, and broke it apart to see if there were any signs of water flow through. There are. Here is a shot of the broken cylinder and the obvious larger granular look to the inside with the discolouration of particulate buildup. Other ceramic materials show no sign of water inside the material like this.
Even so, I wonder how long it is good for as the particulate builds up inside the cylinders it must block water flow the same as a fine sponge would as it's pores fill with material. In the case of a loose group of cylinders you would never really know as the water will just pass around them anyway and the flow through is going to be minimal, even initially.
I guess I still side on the "don't waste your money" side of the argument. If I recall, this stuff is around $10 for a small pouch whereas huge amounts of polyester floss can be had for that price and be as good a bio-film support medium.