What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
Synodontis petricola come from Lake Tanganyika, which has a GH around 400-450ppm and a pH around 8.5.
Basically no you should not have Synodontis petricola in with lemon tetras or even platies. Most rainbowfish will be fine in really hard water and mollies would be ok, but not tetras.
Synodontis petricola also live in groups and should be kept in groups containing at least 6 (preferably 10) or more individuals.
