My 6 feet long tank, what marine system should I choose?

Hi.

I have now got a 6 feet long 150g tank. I was wondering which system should I try to keep, marine or fresh water. I honestly want to keep marine but the fear of loosing fish is confusing me.

Any encouraging advice?

Thanks.

I did an experiment and here are the results.

I filled three 5 gallon containers with saltwater from the beach on 4th March and put all of that water in a 20g tank. And there where 2 larvae of unknown crabs just under 1.5mm in the collected water. Few moon jellies all under 3mm and lot of marine planktons. 2 small fishes 6mm long. And lot of small shrimps. Only the 2 crabs are alive now and are bigger than 1cm I guess.

3 months ago I bought 2 damsel and 1 goby and put them in a quarantine tank 5g in capacity. Instant Ocean was used to make the mix with distilled water. All fish died in days.

Also 3 months ago I got 1 green hermit and on its shell were 3 live barnecals. All of them are alive and thriving I guess.

2 weeks ago I put in there 4 zebra hermits from the beach. Two 6mm long ghost crabs [I guess]. All are alive.

WHat do you guys think of my experiment.

I just wanted to see if creatures caught from the beach can live in aquarium with the water collected from the beach.

There is a clam that I brought back 2weeks ago. Just want to know what it would eat. There are white small things under 1mm in length swimming just under the surface. What can they be.