I would suggest a decent shoal of neons - 12 or so and add a shoal of at least six of a corydoras species, plus maybe a pair of dwarf cichlids such as any of the apistogrammas, German Blue rams or Bolivian rams.

They are much too small for it to be mating behaviour; remember their adult size is at least 3 feet long. They are naturally shoaling fish so I think that is all this is.

They do much better in groups of six or more if the tank is the right size. Males will spar with each other though so it's good to have plenty of plants they can escape into if it gets to be too much.

I think the red patch is from bumping the sides of the tank probably, maybe treat with an anti-bacterial to ward off infection. Have you tried offering him live foods?

No it doesn't. The only way is to use an RO filter (or buy RO water at a shop).

The effect of high gH depends on what fish are kept in the water. Those that like hard water such as Malawi and Tanganyikan cichlids, livebearers like mollies, guppies, platies, and swordtails will all be fine. Fish that prefer soft and acidic water such as most tetras, rasboras, South American cichlids etc will suffer long term damage to their liver and kidneys and osmotic stress suppressing the immune system and leaving them susceptible to disease.

The only way to lower gH successfully is by removing some of the mineral content. This is best done by using a Reverse Osmosis filter which strips all solids out of the water. The correct amount of minerals for the fish being kept then needs to be added back in, either as powdered commercial preparations or by mixing the RO with a proportion of ordinary tap water to get the desired gH and kH. There's an article here on the use of RO water: https://www.fishkeeping.co.uk/articles_109/ro-freshwater.htm

I'd give it a couple of weeks to be sure the tank is infection-free before re-stocking. Quarantine is advisable, but I appreciate it's not always practical. A simple and cheap solution to quarantining is to use a plastic storage box (the "Really Useful" boxes are good). Get an appropriately sized heater for it and a small filter. When you need to use it, take a bit of the media from your main filter to put in the QT filter which will give you some beneficial bacteria to cycle the fish waste. When quarantine is over you can pack it away until next time.

He will be a lot happier in a group of six or more of his own kind but should be OK short term if you can't do that immediately.

Hi

Can you post test results please, for ammonia, nitrite, nitrate and pH? Also size of tank, list of fish and other creatures in the tank, how long has it been set up and how often do you change any water?

This may be a simple bacterial infection so I would treat with Esha 2000. If that does not help it may be something more serious so do the extended course (5 days) of Esha 2000 and then get back to us if there's no improvement. Fingers crossed.