I’m not an expert woodworker but I did have aquariums when I was young.

A 20 gallon aquarium, all set up, can weigh more than 200 lbs. The stand will be under quite a bit of stress and if it fails…

One of my first non-trivial woodworking projects was a stand for a 20 gallon aquarium but I didn’t build it from scratch: I revised an existing table with stout legs. It’s a long time ago, but the legs were tapered and maybe 3"x3" at the top, or maybe even 4"x4". The aprons were maybe 1/2"x4" with mortise and tenon joints into the legs. The top was 3/4" plywood. I don’t know what the wood was, but might have been pine. It supported the aquarium fine, but it was much stronger than your design due to the stouter legs and more substantial aprons. The design was something like Build a Sturdy End Table, except for the mortise and tenon joints instead of dowels to join the aprons to the legs.

I would be hesitant to put a 20 gallon aquarium on a table of your design. The legs and bracing seem a bit weak. If the joints aren’t very tight, if it starts to go over, there might be excessive loads on the brackets and pocket screws, and it might collapse. It might be fine, but I would definitely loose sleep. I had a 20 gallon aquarium fail once (the aquarium, not the table). It’s a lot of water, a lot of mess and very hard on the fish.

If it were my first serious project, I would look for a proven design, specific to holding the weight of an aquarium. There are many designs available, some with nice build videos. All that I have seen are more robust than your design, even for 10 or 20 gallon aquariums.

If I were going to proceed with something like your design, assuming soft pine frame, I would use 2x4 or larger for the legs, wider aprons and angle braces at the corners.