Oliver, a short reply:
1. The cave – yes, the photo is really bad, I hope to identify it correctly – is totally improper for Paros. It is a) too big. Paros prefer small caves. Mostly the smallest available are best. There are caves for small catfish on the market with openings of two or at most three centimeters that are conveniant; the smallest species (ornaticauda and parvulus) like them even smaller (opening 1 to 1,5 cm). The darker it is inside the better. Take a torch for inspection.
2. The most important part of the cave is b) the roof because the eggs are placed there. The roof therefore must be quite horizontal to prevent the eggs from gliding out. I don’t think your cave will match these conditions.
3. The colour of the gravel is of minor importance. Dark gravel is sometimes better, but we have many examples of Paros successfully breeding in tanks wirh light gravel.
4. More important is to use leaves on the ground and good wood from bogs. You don’t need a thick layer of gravel because a tank with a pH of 5 or 4 will tolerate only a few plants. My tanks have a layer of gravel of less than one cm.