Highlights from the Wikipedia article on Pain in fish

The central nervous system (CNS) of fish contains a spinal cord, medulla oblongata, and the brain, divided into telencephalon, diencephalon, mesencephalon and cerebellum.

Studies show that fish exhibit protective behavioural responses to putatively painful stimuli. When acetic acid or bee venom is injected into the lips of rainbow trout, they exhibit an anomalous side-to-side rocking behaviour on their pectoral fins, rub their lips along the sides and floors of the tanks and increase their ventilation rate. When acetic acid is injected into the lips of zebrafish, they respond by decreasing their activity. The magnitude of this behavioural response depends on the concentration of the acetic acid.

Early experiments provided evidence that fish learn to respond to putatively noxious stimuli. For instance, toadfish (Batrachoididae) grunt when they are electrically shocked, but after repeated shocks, they grunt simply at the sight of the electrode

In a 2007 study, goldfish were trained to feed at a location of the aquarium where subsequently they would receive an electric shock. The number of feeding attempts and time spent in the feeding/shock zone decreased with increased shock intensity and with increased food deprivation the number and the duration of feeding attempts increased as did escape responses as this zone was entered. The researchers suggested that goldfish make a trade-off in their motivation to feed with their motivation to avoid an acute noxious stimulus.

We could go philosophy 101 and wonder if you see the same color blue as I do, maybe yours is red? It is easy to say that's immposible to know, but that ignores everything science understands about visible light spectrums, cone recpetiors in the retina and the genetic markers that lead to color blindness.

Fish have nerve endings, they have brains that can process stimuli and their reactions to human standard "painful" stimuli is identical to our own. What reason is there to even doubt they feel pain simmiliar to our own?

Why wouldn't it be reasonable to assume that fish interpret pain as painful? They have a nervous system, inflicting pain on them will trigger a response in the nervous system, this response is most likely similar to the response in humans, that is the pain response is to avoid/remove whatever is causing the pain.