Treating Dropsy

I think you just have a weak batch of guppies. It would be interesting to know how the rest of that batch fared in the shop - did they all live long enough to be sold, or did the shop have deaths as well?

Your filter - what make and model is it, and what's inside it? There are ways to customise filters. If the matrix you added is a chemical medium, it will stop working when it gets full. This kind of medium is not advisable long term.

The Matrix I added is this one (I don't think it's a chemical one, right?): https://www.amazon.co.uk/Seachem-Matrix-Bio-Media-250/dp/B0033Q1U7A/ref=sr_1_4?ie=UTF8&qid=1549617891&sr=8-4&keywords=seachem+matrix

I added all 250ml into the tank, some in filter and the rest outside (next to filter) to see if I can grow bacteria that can kill nitrates.

If you did a fish-in cycle with the guppies, that would have weakened them. Did the platies or gouramis go through the fish-in cycle?

Everything on that list will grow bacteria. The only one you don't need is carbon (that's a hang over from decades ago) but even that will grow bacteria on it. The link says you need to replace the cartridge every 4 week - that just means the carbon containing bit. Carbon gets full and stops working which is why they say to change it.

The best thing is to replace it with a layer of sponge.

Sponge, foam and bioballs should just be washed once every couple of weeks in old water taken out during a water change.

They tell you to change the cartridge every 4 weeks because carbon gets full. But you don't need to use carbon full time. You can either leave the cartridge there until it falls apart or cut a slit in the bag, remove all the carbon and stuff the bag with filter sponge, any make cut up to get it in.
The risk is that the carbon inside the bag will remove any medication you might add which is why medication instructions always say to take out any carbon media. Even when it's been in the tank a long time and is full, there is a theoretical possibility that a medication is more strongly taken up by carbon that what's already on it so the other stuff will come off and the med stick to the carbon instead.

Do you get much brown goo on the media between washes? If you do, that brown goo is a "nitrate factory" so you need to wash it off more often - every couple of weeks.

The filter floss comes without carbon so on my next change I will order those. For now, I will see if I can remove the carbon. Is this the small balls inside the bag? Should I really insert filter sponge after removing carbon?

Quote from: uv on February 08, 2019, 11:05:37 AM

The filter floss comes without carbon so on my next change I will order those. For now, I will see if I can remove the carbon. Is this the small balls inside the bag? Should I really insert filter sponge after removing carbon?

The carbon is inside the cartridge. It is not needed on a day to day basis. It's uses are to remove medication after treatment finishes and to remove the colour that leaches out of wood if you don't like it. It is a hangover from decades ago when water changes were done only a couple of times a year. Such infrequent water changes made the water turn yellow and carbon was used to remove this yellow. Now we do water changes a lot more often we don't get yellow water but filter makers still include carbon in their filters.
Since you do have a fair amount of biological media already (Matrix, bioballs, sponge) it would be simpler just to leave the bag empty and use it for its other purpose of catching fine bits in the water.

Add eSHa 200 to the main tank. Use Epsom salts in a small tub (an old ice cream tub is the right size). You can add eSHa 2000 to the main tank and give the affected fish twice daily dips (half an hour at the longest each time, but if the fish becomes distressed remove it immediately) at the same time as the eSHa 2000 is in the main tank. After the half hour dip in the Epsom salt bath, the fish should be returned to the main tank. Fish should not be left in Epsom salt baths for more than half an hour.

If you have a hospital/quarantine tank, add eSHa 2000 to that and put the fish in this tank between dips. Hospital tanks are just small tanks that a sick fish is moved into to be medicated and this stops healthy fish being exposed to unnecessary medication.