If the others are breathing normally, I would suspect gill flukes (or perhaps a gill infection) making it harder for the affected fish to obtain oxygen. Low oxygen in the water is likely to affect all the fish of a species equally. I would try treating for flukes.
I would reduce the lighting to start with. Brown algae (diatoms) usually go away after a week or two; people can give any advice they like and it will always seem to work. :)
I would also add more plants, especially fast-growing stem plants. Algae seem to have a hard time when there are lots of...
Zero nitrate is not a useful goal.
Happy and healthy fish is a better goal to aim for.
Fish like space to swim around, the choice of interacting with their tankmates or keeping away from them, and a rich environment to live in. No amount of filtration will add that to a tank that is too small.
And some information about it, and anything that's changed recently.
Maintenance and water changes? Tapwater conditioner?
What test kits are you using?
Are the other fish looking and behaving normally?
WCMMs look better in a decent-sized tank, and they will be happier.
White Clouds love to swim about, dashing from end to end of the tank and chasing each other around, in and out of plants and round the hardscape.
You could post a poll to ask people what they use.
I don't remember any-one who uses active substrate claiming that they grow plants that users of inert substrates cannot grow. Although the manufacturers make some bold claims. :)
I am given to understand it's difficult to extend a concrete pond.
I would consider building a new pond adjecent to the existing one as an easier option. If you build the new one a bit lower, you could link them with a waterfall. Just another idea for you. :)
Pair for these, harem for (most) Apistogramma spp.
That depends very much on the tank decor. They like hiding places/caves which are out of sight of each other, eg with plants or hardscape blocking the line of sight from one to another.
Apistogramma cacatuoides is a good first Apistogramma and...
That temp alone shouldn't be a problem, but warmer water holds less dissolved oxygen. It also raises the fish's metabolism so they need more oxygen. It you can't reduce the temp, you could try increasing the aeration.
I've kept White Clouds in a greenhouse which got much hotter in summer, but...
I find that hard to believe. I have kept both, and I wouldn't try keeping them together. Rainbows can be aggresive, and are much bigger than Bolivians,
I would think about Keyholes (as @Conort2 said) or Laetacaras. These also will be safer with your plants.
Or Nannacara anomala. :)
Quite a few people do similar, calling it a 'dark start'.
That sounds good; some people use ammonia, but I think a range of different organic matter would be better. It might give rise to a more varied population of micro-organisms.
I did a similar thing, slightly smaller, at 170 litres. I used 2x2 battens, and these fixings;
The screws are thinner than yours, as the fixing goes into an 8mm hole, so maybe 6mm actual screw diameter.
Three screws across the back and two on each side (into good sound bricks of course)...
I hope you didn't think I was having a dig at you @Matt1994 , you are absolutely right for an aquarium forum. But it I thought it sounds like @jennnndawg is studying plants for their own sake, not in an aquarium context. I hope s/he will be back and perhaps tell us more. :)
The 30ppm mentioned by @Matt1994 applies to aquariums with fish, shrimps, snails etc. It is generally considered a safe maximum for livestock.
In the absence of animals higher levels can give faster growth, - some people do this in a newly set-up tank to get the plants off to a good...
Have the plants been slowly deteriorating since the move? Or was there a sudden die-off?
I suspect the plant problem may be causing (or contributing to) the algae problem.
I would put some cheap stem plants in, and see if they thrive and whether they help with the algae. If they do, they can be...
But Oscars like to lurk in their spot, whereas White Clouds (for example) like to dash about from one end of the tank to the other, chase each other in and out of plants, round and round hardscape and so on.
My late father knew Jenny Owen, who identified (with the help of her university colleagues) 2204 species of insect in her (not huge) garden, including 60 not previously recorded in Britain and six that were new to science. And lots of other creatures too.
She wrote a book, Wildlife Of A Garden...
It will be fine for a planted tank without CO2 injection. If you are going to use CO2 you will probably need more flow. Not necessarily from a bigger filter, - you could add a powerhead for the extra flow.
Commercially bred Neons are quite adaptable, pH7 or 7.5 would be OK. They are also very often weak and in poor condition/health. You might have to shop around a bit to find some good ones. Green Neons usually seem to be in better condition so you might consider those as an alternative.
Species with pendulous flowers and smooth fruits are now Brugmansia. The ones with upright flowers and spiny fruit are still Datura.
Normally pollinated by hummingbirds, but I wouldn't be surprised if bees or moths could do the job; however, natural seed dispersal is believed to have been by...
Hobby Protogen is dried, dormant infusoria. It has worked well for me, but can be hard to find. If your LFS has it. I would recommend getting some.
Liquifry also works, but you have to careful not to use too much as it can foul the water; little and often is ideal. Most aquatic shops stock it...
You should be aware that some plants, under some conditions do not tolerate Glut.
Best known is Vallis, which dies after dosing with 'liquid carbon' for many people. A few have had it survive though, so perhaps it depends on water parameters of other factors.
People have also had various mosses...
Are those leaves brown in themselves? Or is it a coating that you can wipe off with your finger and thumb?
Are the other plant species affected in the same way?
What are the brown spots in the pic?
What is the flow rate of your ffilter?