The best you can do for her at this moment in time is to get/keep water at 0 ammonia, 0 nitrite and <20 nitrates (or no more than 20 above tap water nitrate level). Black is often a sign of ammonia burns.
I'd also ensure that you feed some veg, just in case there's a blockage internally that accounts for the floatiness. Take the shell off a pea, cut each half into tiny morsels, and feed that.
She possibly looks slightly bloated on one side - this doesn't necessarily lead on to anything but I'd monitor her closely. Do you, by any chance, have eSHa 2000 or Waterlife Myxazin to hand? If the situation deteriorates, especially if her scales stand up like a pinecone, then it may be worth treating with that. Melafix is fine (I've used it successfully) but, with labyrinth fish such as gourami, you need to be all the more careful *not* to overdose - err on the side of underdosing if at all possible.
Ammonia levels between 0.5-0, and my gourami has gone a bit clumsy. It’s floating up to get air occasional but staying at the bottom and kind of floating about, that one and the male gourami with her have both had black stuff on them which is really visible today, and the female as a striking white line on her fin/back, what could this be? It’s not looking good for her I’ve added melafix anyway but want to prevent further sick fish.
This week, the ballast to my lighting died and I opted to get https://www.aquael-aquarium.co.uk/prod ... ting/leddy-tube-retrofit/ 8W Sunny as this was the only option to fit my existing tank lid - you can see the colour spectrum distribution on this page; it definitely doesn't do justice to my (last remaining) harlequin rasbora and has rendered her a copper colour.
The cardinals look much the same under the new light as they did under the previous light.
So, if you can find some form of lighting more akin to the former, that might help you.
Fluval is a good brand of filter and I liked the U2. Eheim is also a good brand, and I use the 45 PickUp in my smaller quarantine tank.
I currently have a JBL CristalProfi i80 greenline - it's a corner filter and I thought it seemed more discreet and less bulky. I like JBL products a lot but regret buying this - it's poorly designed and floats right out from the bottom, trapping fish and nerite snails behind it; I bought waterproof magnets to address this but it's extremely cumbersome when doing filter maintenance. OASE Bioplus 100 corner filter might be better - I've heard favourable comments generally about their products.
One problem I've found with a lot of filters is that tiny fish can get trapped behind them easily (eg ember tetras, neon green rasboras, pygmy corydoras). This can be addressed with filter wool squashed behind it.
Undergravel filtration used to be popular and it's still possible to buy undergravel filters - but, if you wanted to grow live aquarium plants, then not a good choice.
Re: Runny and cardinal schooling only during day.
Good to read what you've stocked with - and lovely fish. Glad you're happy with the tank and enjoy what goes on in it.
I have noticed that behaviour changes over the course of a fish's/shoal's lifetime and depends on many factors such as layout, decor, other fish (species and individuals) in the tank, and a lengthy list of others - every permutation has occurred at one time or another over the course of their lives.
Rummynoses in particular are known to be shoalers. Cardinals are known to explore the substrate for food. At the moment, in your case, it might be that the fish feel more comfortable and confident to do their own thing after "lights out". The situation may change over time - there's never a dull moment in fishkeeping.
I have some relatively young cardinals (8 months old) and they are still wary of me when I approach the tank, shoaling together. Once they can see me at a distance or if I sit still for a while beside the tank, they do their own thing, including lots of pecking in the substrate and on the wood for food. Their behaviour is changing too - the males have started taking an interest in the females and so pairing off into smaller groups, while the females are more concerned about fighting over food.