It's a pretty interesting concept... I think undoubtedly it's kinda ponzi game. I don't want to guilt you, because you have clear rules and each player should decide if he wants to play and probably lose money or not. The main weak point to me is that developers' fish is the biggest fish in the game. It's not fair enough imo.
Now the biggest fish is fish with id 7, size 0.5194 eth, it belongs to the wallet 0xb5c518ab9b41080af0d9df48d9ecbf1f098a44d0, we have nothing to do with it.
The player with the biggest fish does receive the most commissions from other players and may not feed his fish, but at any time any member can create a slightly larger fish and then the id 7 fish owner will have to support the growth of his fish, otherwise he will be bitten. Also, if the participant with the biggest fish decides to leave KillFish.io, he will leave part of his fish to feed the remaining fish, so the presence of big fish is beneficial for all players. KillFish.io is arranged so that the big fish really gives an advantage in the game, but its owner must understand that with great power comes great responsibility.