I have not tried Marine tanks as reading up on them the water is very sensitive to keep the right salinity. I do enjoy watching the films on the Great Barrier Reef, I think there is a series running now with David Attenborough. I have tried tropicals in small tanks but gave up with cleaning tanks and again fish are very sensitive to changes in their environment. As a kid my father had a pond and my love of wildlife developed from there. It is great that so many schools are putting their pupils on a path that encourages them to respect the natural world. It was natural that when I got my own home I would build a small pond with goldfish for my kids to enjoy. When I moved to our present house which already had a pond I decided on a dedicated Koi pond and built one with blockwork and fibre glass to seal the cement as the later is toxic to fish. As you know Koi are sensitive to their water quality as well so a filtration system is essential. Mine is built in as part of the original pond with chambers to move the water on and ""clean" it. The first takes all the big waste from a drain in the bottom of the pond next are brushes and matting to clean smaller particles an then more matting to encourage biological bacteria and the like to break down fish waste. The final chamber has pump to keep the water circulating and constantly clean the pond to give clear water to best view the fish and keep them healthy. You will probably know that some koi are extremely expensive and sold for thousands of pounds all over the world. They originate from China and more famously Japan. There are numerous varieties of koi usually distinguished by their beautiful colouring or their scales Some have no scales as such and are smooth. I have never paid more than £20 for a Koi but that is not to say you are not getting a beautiful fish. I have always bought 1 year old fish and brought them on. Some I have are 25year old and have grown huge. They are a member of the carp family and I have 2 mirror carp and 2 grass carp in with the koi. They are easy to keep, water quality is everything in a relatively small pod. They will live in very large ponds lakes without filters just like other fish. To get back to fish tanks, yes it is very relaxing to watch fish and dentists and hospitals often had a tank. Fish debunk the seven second memory theory of a goldfish as they see me coming to feed them so must relate to it, All animals have much more intelligence than some humans give them credit for. Enjoy your hobby