I last went about 3 years ago. It's a small place, you can spend around 10-15 mins having a wander round. Try to tie it in with their sardine night (if they still do it?) It was every Friday. Served out on the wooden terrace right on the waters edge. Pretty much all you can eat with salad and potatoes. Think it was around €14 each then. No toilets, you need to nip up the cliff pathway to Hotel Tanit.
I agree with @Amigo . Although it's reasonably priced per se, the cost of admission isn't really justifiable given how small it is. But go one evening and combine with sunset and their all you eat sardines - then definitely a good idea. Dusk swims are also great there. In the sea, not the aquarium
It's a great little place. I always sit with my feet in the water and have a couple of drinks. perfectly quiet and away from a busy San An. Walking back round I always visit the little reggae bar as they do a huge range of beers!
When we used to stay at Sol Bahia/Gran Sol or Blau Parc, we always used to have a wander round for a drink or two. Really nice place to go & just chill out. In all the times we've been, I've never actually looked round the aquarium!