I suggest that you work several years for a established and reputable pond company. This assumes that the employer is well leaned and knowledgable. IMO there are lot of naive pond "jockeys" out there claiming to be pond managers. Ask and learn all you can while working on the job, then start gradually building customers until you can support it full time. It helps to have sound and good biological training with some chemistry background and learning experiences. You will and should learn that pond management business is not just killing plants and stocking fish. Part of your job should be educating the customer. Few do this IMO because the Pro Pond manager does not have good sound ecological knowledge in the first place

This on the job training would allow you to know many of the answers to questions you are asking and MORE. It should take lots and lots of learning and knowing all the thousands of the critters and plants involved, their ecological niche in the pond system and how having and not having them present, impacts the whole pond ecosystem and the associated food web. Ponds are a complex biological and chemical soup and a manager should fully understand what these chemicals are, how they interact, how to measure them and react and are impacted by the chemicals that you apply.

Be responsible that you do not turn ponds into chemical dumps that can definitely impact the later value of the pond for the owner. Ponds that just look good with dye and are all plant-weed free can easily become chemical dumps and continually building up unhealthy chemical deposits.. Most ponds are not toilets and do not get flushed regularly. What happens to the chemicals that are applied over time and their environmental fate? The pond ecosystem has to somehow "deal" with all the added chemicals. Know the ecological fates of the chemicals that you are suggesting, promoting and applying. Professional pond managers should IMO be legally liable of what they do to the long term health of the whole pond ecosystem. That day will come as the green society advances.