When they start a coral restoration project there often is no coral reef there anymore and so they use coral-rubble or something they call a reef star (also sometimes other recycled metal objects) and so there is no fish ecosystem at that location. Like any other ecosystem, as some fish move in some will be predators and others will be prey in a food chain starting with fish that eat plants and so on and then fish that eat fish. The whole point of an ecosystem is that this forms a more or less stable system over time.

Reef Star: https://buildingcoral.com

Other than that, the scientific paper I linked to is open and writtem in fairly clear language.

> Surveys of fish communities

> The family Pomacentridae (damselfish) was surveyed regularly throughout reef deployment. The high abundance of damselfishes on coral reefs (up to 50% of reef fish communities18) facilitates adequate statistical power to test for differences in community development, and their non-cryptic nature allows accurate surveying with minimal disturbance. Visual surveys by a SCUBA diver (T.A.C.G.) were used to monitor communities of juvenile damselfishes (those that had settled on reefs following a pelagic larval phase in the current season). The observer and dive buddy remained at least 1 m from the reef during surveys, in order to minimise disturbance to the community. Each reef was surveyed 10 times throughout a 40-day period that started immediately following construction, with 3–9 days between consecutive surveys. The start of the survey period on each reef was staggered across a total duration of 10 days (i.e. construction and surveying of the final reef started 10 days after the first reef was built), to allow a single observer (T.A.C.G.) to complete surveys at the same experimental time point. Surveys took up to 1 h per reef; staggering was therefore necessary to allow for surveying of 33 patch reefs at the same experimental time points. Deployment order was counterbalanced such that the same number of reefs within each treatment were constructed and surveyed on each day.

> After 40 days, the whole fish community on each reef was surveyed by dismantling the reef piece-by-piece. An observer using SCUBA (T.A.C.G.) checked each piece of rubble thoroughly, using dilute clove oil and a hand net to capture all juvenile fishes on the reef. The dive buddy kept a continuous watch for ‘stray’ fishes that attempted to escape across the sand flat or burrow into the sand during the survey process. All reefs were double-checked for missed fishes after surveys were completed; to our knowledge, no fishes were missed from surveys. All fishes were identified to species, except in cases where uncertainty meant that identification was only possible to family or sub-family level. In these cases, fishes that looked similar were assigned as the same species (e.g. ‘Unknown goby 1'); this was the case for 4% of species, whose members together constituted 9% of the total abundance. Adult fishes were excluded from analyses, as their larger home ranges mean that they do not exhibit fixed associations with reef habitat to the same extent as juveniles40. After the experiment, all fish were released alive onto neighbouring reefs.

You could email Tim if you have more questions.