One fish two fish red fish blue fish

My five-year-old is a bit...obsessive. When he was a baby it was letters, when he was a toddler it was trains, we had a brief flirtation with the planets, and now he loves fish. For his birthday last year, his aunt got him a little aquarium and a betta fish.
The fish was blue, and my son named him "Bluey." That's how he rolls - he comes by his inner engineer very honestly, and one trait that shines through is an incredible lack of creativity in naming his critters. Generally the formula is "dominant descriptor"+"-y"=name. We have Bluey, Trouty, Whaley, et al.
Bluey lived happily in his aquarium on the kitchen counter. In July, my husband and I separated. In August, we'd already planned a family trip to the beach, so I asked D to come over and water the plants and tend the creatures. He assured me while we were gone that he was watering and feeding as needed.
When I got home with the boys (after a 4.5hr drive with no AC) I unpacked them and the car, unloaded the cooler, and wrangled them to bed after dinner. As I was drifting through the kitchen afterwards, I happened to glance at the aquarium...where there was about a week's worth of food piled on the bottom. Oh, shit. I popped the top off, and there was a Very Dead Fish. Eew. Ziploc/trash.
The next day was spent in frantic communication with my attorney, because on top of the fish fiasco, that Sunday was when I discovered the emotional sewer backup in the foundation of my marriage. [that's it for bad metaphors, I hope] During one very tense text exchange, I told my very VERY estranged husband that if he really wanted to parent, he could go replace the goddamn fish. He went to PetSmart, where they have cups with male bettas. The water is dyed blue to look pretty. Most of them are blue, but not all.
He bought a red fish. To replace a blue fish. For a child who misses NOTHING. The next day when we got home from school, there was a great commotion from the kitchen. I usually try to be honest with my kids, but i just couldn't tell him that his dad left and his fish died. Among his first comments was, "But Mommy, I can't call him Bluey if he's red!"