Congratulations. I hope you are on your way to raising some baby angels.
I am guessing this is the first time your angels have spawned, so I will start from square 1.
First a 15 gal
tank isn't really large enough for your angels. The pair should be in their own tank - preferably a 30gal tall tank. Angels are very tall with really sensitive fins. A tall tank provides them the room they need. The reason for their own tank is a mated pair can become extremely aggressive when they have eggs or fry. I am surprised yours have not been harassing the tetras. They also tend to eat their fry if they think there is any threat - like other fish in the tank.
The angels will normally lay their eggs on a vertical surface. Give them some things to use such as a piece of slate, a pvc
pipe or long leafed plants; locating them as far away from the filter intake as possible. (If the eggs are close to the filter intake they may not get fertilized.)
Assuming you want to let the parents raise their own fry (or for the first several spawns start learning how) you can leave the eggs and fry in with the parents. The parents will usually move the eggs and wigglers around. The more surfaces, the better chance they won't lay eggs on the heater or filter intake. Place a fine sponge over the filter intake tube, otherwise the fry will get sucked into the filter.
Eggs that are viable will be an amber color. Any white eggs will be eaten by the parents or just fall off. As long as the parents are doing their job there is no need to remove these eggs. The parents will take turns fanning their eggs to prevent the development of fungus. After 2-3 days the eggs will develop little tails and become wigglers. The fry have a sticky thread on their heads. You may see the parents take a bunch of wigglers or fry into their mouths and then spit them out onto another surface. The wigglers will stick to the new surface.
5-7 days after hatching the wigglers will start to free swim. They will swim in a cloud around the parents. The parents will still snatch them up in their mouths and spit them back out. Don't get alarmed if you see them doing this, it's normal. The fry will need to be fed at this point. The best food for them is BABY brine shrimp. (I made the mistake of using full grown brine shrimp, about the same size as the fry. In their frenzy to gobble up the treat, the parents didn't stop to distinguish between shrimp and fry.) Feed the fry about 3 times a day, eventually mixing in something like Hikari First Bites.
For whatever reason angelfish will eat their fry if they perceive any threat. Consider covering all the sides of the tank with black paper and locate the tank in a non high traffic area. Put some vegetation in for the fry to hide in - java moss and floating hornwort are good choices. You can keep the fry with the parents until they start to look like angelfish (2-4 months) at which time you can move them into another tank. Or move them sooner if the parents become aggressive to them. (Keep in mind the parents will continue to spawn new eggs every 2-3 weeks.)
Oh, using a substrate is not an issue. The parents won't be bothered by a substrate.
I didn't address if you want to remove the eggs and try to hatch them on their own. That adds a whole new dimension of work. And, I prefer to just let nature take its course (with a little help. :P)
Hope this helps you out some. Good luck with the babies.