I figure there are several reasonable explanations for his absence:
A) He decided to test out some of the electrical work and his house short-circuited, cutting off his connection to us.
B) His group of 15 Arowana's kidnapped him and sold him into human trafficking.
C) Neptune, God of the Sea, summoned a hurricane as the only means to fill his tank, and he is cleaning out his garage again, now filled with debris as it is.
D) He realized this far into the project that he's lost interest in fish, and is now developing the room into a lion den, and no longer cares about us.
E) He built the tank from the wrong direction, and is now stuck inside it, watching the world through some glass windows.
F) He went whaling, hoping to catch a reasonably-sized Blue Whale for his new aquarium.
G) Or maybe he's just busy, had to go on a trip to pick up a supply of some sort, got the flu, or needed a break.