Russel,
I am new to this site and to Breeding Syno's but i have been breeding cichlids for years and thought i would share my method of artificial incubating with you and hope it can help you with your Petricola.
I use a plastic seive either in the tank or in a bucket of water with a heater in it and an air stone. I use it for my Frontosa Kitumbas (Zaire Purple) which are pretty rare in the UK and are very difficult to breed so when i get eggs they are like gold dust and using the Seive method i have nearly 100% hatch rate.
Using the seive allows poop (as you call it) to go through the small holes but not the eggs. Also when tumbling with air the eggs always come back down to the bottom because of the shape of the seive and do not end up around the edges and not getting water flow. I have some pictures on my website that show Frontosa fry 13 days old being incubated this way. Hope this helps you.
www.zaire-purple.co.uk