I would go in person and explain who you are and what your credentials are. I have no problem confronting these problems because I explain who I am (batrachologist/herpetologist, director of NEHS, deputy administrator of the Frog Forum, former employee of both MoS & HMNH, former worker/manager in the pet industry itself, and a major keeper of fish, amphibians, reptiles, and various invertebrates). When talking to them don't get emotional and explain your points clearly. Do not go on the attack, this will just make them defensive. Go in as an educator and not an accusor. Say something like "You know, I noticed that you were housing Spanish-ribbed newts with frogs. Did you know that ALL newts are toxic and so are most frogs. Also, all amphibians have porous skin and easily absorb toxins. So basically, by keeping these two species together they are slowly poisoning each other. You could end up with very unhealthy or even dead animals. I would seperate them as soon as possilbe." For now, drop the algae issue. Algae is harmless, it just looks awful to most people. Just go after clear issues of animal neglect or ignorance of proper husbandry. Make sure you know what you are talking about, before opening you mouth. You don't want to come off like an idiot, because everything you say after that will not be taken seriously. Most importantly, be friendly, act like you are their friend and that you are trying to help them.

Most of the time, what I say is taken seriously. I can only think of one incident in recent memory where my advice was scorned. I was in a Petco and their rosy boas were in a sopping-wet substrate as were their hermit crabs. From the hermit crab enclosure was the unmistakable smell of rotting flesh. I tried to explain that they had a dead hermit crab somewhere in the cage, but I was told that's just how they smell. The employee was very defensive. I was told by another employee they had a new guy who just started and he was taking care of the reptile department, not really a valid excuse, but better than arguing with me. I have been working animals longer than these people have been on the planet. So be prepared for a defensive attitude, even when you go out of your way to be nice.