There are a number of hits on the subject "do fish need vitamin D". There are some references to aquariums and glass cutting down UV that made me determine I might get some benefit from additional Vitamin D in my own diet (I have a medical condition that requires me to get a lot of light)and digressions about eating fish but there were interesting articles about the lack of a need for Vitamin D and C in fish. It seems if the water is rich in Calcium and Phosphates, they don't need Vitamin D.

One short but informative site reads:

"Is vitamin D redundant in an aquatic habitat?
Rao DS, Raghuramulu N.
Source
National Institute of Nutrition, Hyderabad, India.
Abstract
Certain fish are very rich sources of vitamin D as compared to most of the higher vertebrates which have insignificant amounts of this vitamin. Not only the teleosts, which possess a calcified skeleton, but also the elasmobranchs, which lack calcified skeleton, contain extremely high concentrations of this vitamin, leading to the speculation that the function of vitamin D in fish may be different from its known classical functions in terrestrial animals. Interestingly, the two most common calcemic hormones associated with Ca and P homeostasis in higher vertebrates are either missing [parathyroid hormone (PTH)] or inactive [calcitonin (CT)] in fish. In fact, these hormones appear to have developed after transition of life from water (Ca-P rich environment) to land (environment poor in Ca and P). Thus, living in an aquatic environment with a continuous rich supply of Ca and P, do fish need vitamin D? If so, does it need to be converted to its polar forms? Additionally what are the functions of vitamin D and its metabolites in fish? Since fish stand between the invertebrates and higher vertebrates in evolution, they serve as a unique model for the study of the evolutionary and physiological significance of vitamin D. Investigations have demonstrated that the source of a high amount of vitamin D in them is primarily through their food-chain (plankton). In addition, it appears from the studies in fish that vitamin D perhaps had no physiological function in the calcium-rich aquatic environment, and its metabolism was essentially for catabolic purposes. During the course of evolution, when life started on calcium poor terrestrial environment, vitamin D became functional and its metabolism, an anabolic one, was concerned with calcium homeostasis."

I have to conclude your fish are not trying to get sunlight for vitamin D and while I guess it is possible there are other reasons, the most likely are that they like the warmth or the light makes them feel more lively.

We a couple of Black Moors in an aquarium. The sun is low enough that the sunlight through the window will hit the lower front corner of the aquarium for an hour or so each afternoon.

When it is quiet in the house and the sunlight hits the aquarium the fish will seek out that corner and just lie on the bottom there.