The easy solution (in USA & Canada) to the problem of a full load panel is to remove a regular breaker and install a "tandem" breaker in its place. A tandem breaker is just 2 skinny breakers that occupy the same space as 1 regular circuit breaker. Yes, a tandem puts 2 separate distinct circuits in 1 slot. Tandem breakers allow you to (almost) double the number of breakers inside your panel. I say "almost" because a tandem can supply two 120v circuits but it cannot supply 240V to a circuit. That's cuz a 120v uses 1 "hot" conductor, whereas 240v needs 2 "hot" conductors (1 from each side of the panel). So although you cannot replace your 240v dryer, A/C, or range breaker with a tandem, you can indeed replace any of your 120v breakers with tandem breakers.
Although a 15A circuit is all you'll need for your new aquarium, there's no harm in installing a 20A circuit since you're going through the same trouble to run a new cable from the panel
drayage los angeles to the tank . The only difference is that 20A will cost you a pittance more, cuz you'll need to use 12 gauge (yellow jacket) wire which costs a few cents more per foot than 14 gauge (white jacket) wire that's used for a 15A circuit. Might as well go 20A, for peace of mind & allow for future needs.