Final answer:
To determine which aquatic species will thrive in an aquarium set to a pH of 6.4, 18 degrees Celsius, and less than 3 mg/liter ammonia, species should be chosen based on their optimal growth pH and temperature tolerances which are close to these conditions.
Step-by-step explanation:
The question pertains to the suitability of environmental conditions for aquatic species in a laboratory freshwater aquarium. To maximize the number of species that thrive, each species' optimal ranges for pH, temperature, and ammonia concentration must be considered. The specified conditions are a pH of 6.4, a temperature of 18 degrees Celsius, and less than 3 mg/liter of ammonia.
Most aquatic organisms are adapted to certain pH ranges and temperatures that support their biochemical processes. Freshwater ecosystems tend to have a broader pH range where life can survive compared to seawater, and because the aquarium in question has a pH slightly below 7, species which prefer a neutral to slightly acidic environment might be suitable candidates.
Furthermore, dissolved oxygen and nutrients are typically the limiting factors for aquatic species, but since the question doesn't state anything about low oxygen levels, we will focus on the pH and temperature parameters. Therefore, species with an optimal growth pH close to 6.4 and capable of tolerating temperatures around 18 degrees Celsius would potentially thrive under these conditions.