I lost my babies

🙁 I woke up this morning to find all my beautiful tank mates: 2 seahorses, 2 clowns and my scooter all dead. As you can imagine I was devastated. Everything else in the tank was in a very bad way, my soft coral was struggling and my Emerald crab, hermit crab and peppermint shrimp and snail were not coping at all well.
I could not understand it, last night they were all very happy. Everything was eating well and brightly coloured and swimming around and then all of a sudden they are gone.

I took all my survivors to the local aquarium store as I didn’t believe I had any hope of keeping them alive if I didn’t and also took a water sample so I could get to the bottom of why I lost most of my tank mates in this horrible way.
Turns out over night my water levels shifted and the ammonia levels went quite high and will have poisoned my poor sweethearts. I pray they didn’t suffer too much.
I still am in disbelief this happened over night suddenly. Especially as I said before everything seemed fine. I got told by the man in the shop that I overloaded my tank with livestock does this sound right? I have a Clear seal Mini pentagon aquarium that holds 9 gallons (39.15litres). Its 15” high and I thought it was a perfect tank for me to get use to my new pets in before upgrading when they got larger.
I use a Fluval filter and heater and also have the fitted lights in the tank hood more for the coral than anything.
Please can you give me you opinions on this setup, my seahorses were very precious to me and I feel so guilty that they have died. I would like to eventually try again however I want to know whether I will need to buy a new tank. I have also heard that a good protein skinner can prevent this happening again so if I don’t need a new tank should I invest in one of those?

Thanks
An emotional seahorse keeper, Charlotte 🙁