Fish rapidly dying

yeah you lost your cycle when you swapped filters. If you are unfamiliar with cycling an aquarium I highly suggest you research the topic. Bacteria is in the filter media that breaks down toxins produced by the fish. This bacteria take time to establish, 4-6+ weeks often. Without it the toxins build up in the water and kill the fish. Not saving the media from the tetra filter caused a large loss of bacteria resulting in fish loss. Your nitrite at 1ppm is high, easily high enough to kill gouramis. It should always be zero, since it is toxic to fish. The presence of nitrite almost always happens alongside an ammonia spike, which again is toxic and can easily kill your fish. The ammonia spike comes first since it is produced by the fish with the nitrite following. Your test strips do not test for ammonia. I would recommend getting an API liquid test kit its what many people use.

Currently I would recommend a large water change. 50% at least to cut down the nitrite levels. You will be doing a lot of water changes for a while to keep levels down until the new filter establishes. 50% every other day if not more. A better test kit will help you determine this. Stop feeding the fish for the most part, like once every 3 days or so to try to slow down the build up. For the future always transfer media when changing filters. Even if you need to shred up the old media and shove it in the new filter it will avoid water quality issues. Its usually recommend no more then 1/3 of the filter media be replaced at one time.

The tankmates may be fine for now but 4 gouramis in one tank was asking for trouble. The pitcus and gar will potentially reach 8-10" in a proper sized tank. Just don't be surprised if smaller tank mates disappear one night.

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.

Jeff.