Welcome to TFF.
The stocking mentioned has issues that are not so much due to the actual fish mass/water volume ratio, but more to the fish species requirements themselves.
Chichlids (angelfish) and gourami should never be combined in the same aquarium, with very few exceptions and large tanks. Basically they are very similar in their temperament, males being highly territorial. Just as you would not have more than one species of cichlid in this tank, you also shouldn't have cichlid and gourami.
On the angelfish themselves, this is not advisable. This is a shoaling species, meaning the fish live in groups. They develop an hierarchy; males are territorial, so there needs to be a good amount of space (primarily in tank length) to accommodate this. In nature the space is basically infinite but in any aquarium it is immediately limited and this affects the fish. A group should be five minimum, but there is certainly insufficient space in a 180 liter (47 gallon) tank whatever the dimensions. The only way two angelfish should be considered for any tank is if they are a mated/bonded pair (male/female), which of course adds issues like spawning, other fish, etc. I would forget angelfish and consider gourami; you could have some lovely Pearl Gourami here. Avoid the Blue/Gold/Cosby/Opaline/Three-Spot/Marble gourami as these are all varieties of the one species
Trichopodus trichopterus and notorious for bad behaviour to each other and frequently other non-gourami species.
If you want either cories or kuhli loaches, make sure the substrate is soft sand. Play sand works very well. Both species will only be at their best with sand, for differing reasons.
Before we start considering other shoaling species, we need to know the source water parameters (GH, pH and KH). There are lots of options, but some will bee more demanding than others when it comes to parameters.
Byron.
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