6.8 is on the low side, so as above said I would go with South American fish.
Now South American fish are broken into two groups for the most part. Those from West of the Andes and those from east of the Andes.
The west ones are more closer to Centrals in water peramaters needs. Ones from the east are the ones that thrive in the low PH soft water.
Ones to look at Sevrums, Oscar, Chocolat, Geos, true parrot cichlid.
Definitely can make a nice setup with these choices.
1) Get 6 to 8 baby Sevrums. In time you should get a pair. Return the rest to the store or sell etc. For the bottom you can get Cory cats or Pictus cats. Get a nice big school of tall bodies tetras as dithers. The Cories/Pictus and tetras will provide a lot of activity, the pair of Sevrums will eventually breed so you will be able to enjoy that with out worrying about fry removal. The tetras and Cories/Pictus should eliminate them.
2) A single Oscar.
3) A group of Geos. Geos are bottom fish and need a fine sand bottom as they are sand sifters. Get some tetras as dithers. With this setup and choice 1 you could add a pink tailed Chalceus as a top of the water column fish. With these it's either a single or a big group. Too few and they think it's a episode of Highlander and there can only be one. But in a group they are more natural. They are in the Tetras family but look and act more like a barracuda.
Now South American fish are broken into two groups for the most part. Those from West of the Andes and those from east of the Andes.
The west ones are more closer to Centrals in water peramaters needs. Ones from the east are the ones that thrive in the low PH soft water.
Ones to look at Sevrums, Oscar, Chocolat, Geos, true parrot cichlid.
Definitely can make a nice setup with these choices.
1) Get 6 to 8 baby Sevrums. In time you should get a pair. Return the rest to the store or sell etc. For the bottom you can get Cory cats or Pictus cats. Get a nice big school of tall bodies tetras as dithers. The Cories/Pictus and tetras will provide a lot of activity, the pair of Sevrums will eventually breed so you will be able to enjoy that with out worrying about fry removal. The tetras and Cories/Pictus should eliminate them.
2) A single Oscar.
3) A group of Geos. Geos are bottom fish and need a fine sand bottom as they are sand sifters. Get some tetras as dithers. With this setup and choice 1 you could add a pink tailed Chalceus as a top of the water column fish. With these it's either a single or a big group. Too few and they think it's a episode of Highlander and there can only be one. But in a group they are more natural. They are in the Tetras family but look and act more like a barracuda.