Do a couple of big (50%) water changes and then start treatment.

If you don't want breeding that rules out the livebearers (platy, molly, guppy, endler, swordtail), but they wouldn't like your soft water anyway.

You could go for a pair or group of angelfish, a school of larger tetras like the phantoms or lemon tetras and a shoal of Corydoras such as Corydoras sterbai . If you don't like angels you could have a couple of pairs of Apistogramma instead. Alternatively, go for an Asian stock list and have a group of honey gourami, a shoal of one of the more peaceful barbs such as the rosy barb or harlequin rasboras and some dwarf chain loaches.

I think the fish probably has a bacterial infection which can be treated with Esha 2000 or Waterlife Myxazin.

200L/4ft really is the bare minimum for a Malawi community. If you drop down to 180L you'd be best sticking to a single species: maybe one of the dimorphic types like Pseudotropheus saulosi to give you two colours.

I'm not familiar with Oase filters, but from a quick look online I'd be inclined to use the 350 on a 180 or the 600 on bigger tanks.

Hi
I think the hole has happened after death, probably another fish nibbling at it.

You really need a considerably larger tank as 18 litres is much too small for mollies, or any fish reallyand you will be constantly dealing with poor water quality and the problems for fish that brings. Mollies need a 3ft long tank, ideally.

Unfortunately, the shop has given you very poor advice. I would be looking to either upgrade the tank or rehome at least one of the JDs. Even one with firemouths may be a problem as in that size of tank a mature JD will see all of it as his territory.

Hi
I'm afraid we don't have any marine keepers currently on the site so you may get an answer quicker elsewhere

Spray bars don't cause pressure on the pump so it's fine to use. Position it how you like, just let it disturb the water surface.

Just another point, but the tank is too small for those fish. A pair of Jack Dempseys need a tank at least 4ft long by 18 inches wide; if you have two males you'll need more space than that. To keep JDs with other cichlids like the firemouths you're looking at a 6 foot by 2 foot to maintain the peace when they all mature.

When a range of temperatures/hardness is given, it's best to keep fish roughly in the middle of that range, not at the extremes (other than short term).

A lot of the information on the net is either incorrect or out of date (especially any American sites) so we usually recommend this site for fish profiles as it's very accurate and is kept up to date: http://seriouslyfish.com/knowledge-base/

There's no set timescale as each tank is different, but typically it takes 2-3 weeks for nitrite to appear :)