This study found that the GERD blocks natural floods that nourished fish habitats making the Nile’s ecosystem less healthy and productive downstream
Another study found that downstream water could become 1.5 °C colder and flow 5–42% slower harming aquatic life

Ecological status of Lake Nasser Khors, Egypt, before operating the Grand Ethiopian Renaissance Dam - Stochastic Environmental Research and Risk Assessment
Lake Nasser is an artificial reservoir that represents the national freshwater reserve of Egypt and has a number of khors (side extensions) along its eastern and western banks. These khors covered approximately 79% of the lake’s total surface area and have an important effect on water quality...

Research has already shown dam building in the Nile has cut river connectivity from over 50% to under 20%, fragmenting habitats and isolating fish which threatens their diversity and survival

How Dams Change the Nile River’s Connections Over Time
Rising dam numbers in the Nile Basin are fragmenting rivers, endangering aquatic life and disrupting ecosystems. A BOKU University study highlights the need for fish corridors and better management to preserve biodiversity and support millions who rely on these waters.

I had also seen one Egyptian researcher say that in just a decade 12 native fish species and freshwater crayfish could disappear entirely, but it looks like Sudan will also be hit hard by the dam, the dam hasnt even been inaugurated yet and there's already been signs so I can't imagine in a few years
It's in Ethiopia's best interests that the war in Sudan continues forever if they want to keep that dam