The tank has been set up for just over 1 month but how long have there been fish in the tank?
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Do not add any more medications. Melafix and Pimafix can sometimes cause problems to labyrinth fishes (Bettas & Gouramis). It can leave an oily film over the surface and the fish suck this in when they breath and it can poison them. Most other fish are fine with it but it is not necessary in this case.
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Do not get any more fish for at least 1 month after everything settles down.
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Do not replace any filter materials for at least 1 month, preferably 2 months. If you want to add a round sponge to the intake strainer of the filter, get a sponge from an internal power filter and put it on the intake of the TopFin. It will give you more filtration and act as a screen to stop fish being sucked into the filter.
You can also add sponges from other brands of filter. You buy any brand of filter sponge from a pet shop and cut it to fit. Then put it in the filter to add filter media.
You don't need carbon in the filter unless you have chemicals to remove and they can be removed with water changes.
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It is my opinion the ammonia levels are causing the problem due to the pH of the water. In water with a pH below 7.0 ammonia is pretty harmless but in alkaline water (pH above 7.0) it becomes very toxic and will kill any fish in the water. The higher the pH, the more toxic it becomes. A pH of 7.6 will make any ammonia very toxic.
Do not try to change the pH because this will stress the fish out more.
Reduce feeding to 2 times a week and change 75% of the water 4-8 hours after feeding them. Gravel clean the substrate too.
In addition to this, do a 75% water change and gravel clean the substrate on any day there is an ammonia or nitrite reading above 0. Make sure the new water is free of chlorine/ chloramine before adding it to the tank.
Make sure you use buckets specifically for the fish. Do not use any bucket from around the house because they may have had cleaning products in that will cause problems like this. If possible buy a couple of new buckets and use a permanent marker to write "FISH ONLY" on them. Keep them with the fish gear, nets, gravel cleaner, etc, and only use those buckets for the fish. Do not let anyone use them for anything else.
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If you are adding liquid bacterial supplements like Fluval Biological Enhancer, you can double dose them every day for 1 week and then put the rest in the fridge, or you can use it all up. Try to add it to the aquarium near the filter intake. Or turn the filter off and pour it into the filter. Wait 15 minutes and then turn the filter back on. Normally just add it to the tank near the filter intake tho and it will be drawn into the filter where it is needed.
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There is more info at the following link if you have a bit of spare time. It is pretty much what I have mentioned above but more thorough.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
If you reduce feeding, do more water changes and stop adding chemicals, hopefully the fish will come good in a few days. However, you will need to continue with the water changes and reduced feeding for at least 1 month, until after the filters have established. then you can increase feeding and reduce the water changes.
As for the male Betta, I would put him back in the main tank and do lots of water changes. If you have concerns about him being damaged by the filter, you can leave him in a plastic container and change most of the water each day. Just make sure the new water is free of chlorine/ chloramine before it is added to the container.
