For me it is the realization it is a hobby not a lifestyle (for me). Had up to 7 tanks due to mts, two 10 gallons, two 20 longs, 37, 60, and 75. Weekend would come by and I would have to maintain all those tanks, this was taking too much time reducing the time I spent hiking and kayaking, things that help my physical health. I mostly keep angelfish and found that the ones in the downstairs tanks not as interactive with me as my fish in tanks located in our family room. In the end I downsized, much to the benefit of our local fish stores, to only 2 tanks 37 and the 75 gal. I can fully service the filters, vacuum the substrate, replace 60% of the water, and prune the plants in 2 hours now. In addition, my angels see us all the time making the servicing the tanks more fun for me because they follow me around the tank as I prune the plants and vacuum the substrate.
Other regret is getting a couple of tall narrow tanks. Anything over 20" tall is a pain to work with and are only appropriate for fish that need that depth. Pterophyllum scalare is not really tall enough to justify a 25" tall tank.
