Hi I'm new to aquariums keeping however after assisting my boss look after her Seahorse Aquarium I have done a lot of research and wish to start my own aquarium.
I have done loads of research but wanted to get others opinions in case I missed anything.
I will not add all these things right a way its just a goal for where I would like my aquarium to look like.

I wish to purchase a tank between 150-300L with a minimum height of 50cm.
I would like to use live rock and a live black sand bed.

- My main feature will be the 2 seahorses (tropical but unsure what species exactly looking at hippocampus erectus)
- I would like 2 Green Mandarin Dragonettes
- An Astraea Turbo Snail
- Bumble Bee Snail
- Feather Dusters/ Fan Worms and Dwarf Duster in the Rock
- Either an Orange Star Fish or Linckia Blue Starfish
- Dwarf Blue Legged Hermit Crab
- and a copepod breeding in the corner of the tank

Coral I have been looking at include:
- Spiny Sponge
- Blue Sponge
- Sun Coral
- Caulerpa
- Yellow Fiji Leather
- Red Flower Tree Coral
- Colony Polyps
- Lavender Mushroom
- Purple Ribbon Gorgonian

With my Tank Mates in mind Believe i will need the following water levels:
Density 1.023-1.026
Ammonia and Nitrites 0
PH 8.1-8.3
Temp 20-26 Degrees Celsius
Am unsure of water flow rate, alkalinity, dKH and any other levels required.

If people could please let me know If i'm making any mistakes or if they have any suggestions.

Thanks Im really looking forward to the challenge of maintaining this tank and would like to avoid fatalities.

That's terrific to hear of a new starter to aquariums! I've only been into my reef tank for about 5 months so I'm sure more experienced people will give a little more insight. I may be new but I still have done a ton of research (just for the record). I'll just lay some insight into the starting of the aquarium and leave the seahorses to the experts. For temp and saltinity you've got that down. For water movement, sponges require strong flow because they filter the water. That being said they also don't like light as much because a micro layer of algea can build on them clogging their pores and killing them. I've got a sponge right now that I'm trying to save, and wish the guy would have told me requirements. So, with strong flow an little light this makes them a difficult coral to keep but if your up for the challenge (which it sounds you are) just take note that there could be a death. Typically water movement should be a minimum of 10-20 cycles per hour. That means with a 100-150L tank you want a minimum of 1,000L-3,000L cycle per hour. I like to have mine at 25x per hour for any sps or hard corals and my sponge. With cycle too, filtration comes into effect. So be sure to add the amount that your filter cycles into the watermovent. With Alkilinity, it should be kept at 4-6mq/L (I could have that identitfication wrong). Some people however do keep it at or around 8. Alkilinity is a mixture of salt which maintains calcium levels (In case you didn't know). Know you didn't mention it but I'll just put it out there. Lighting is a huge aspect of a reef tank and is often overlooked. I've used Exotic LED strips because there affordable, great lighting, cheap to run, and last long. I'll leave that to you though because you can choose from T5, Halogen, T5/Halogen, LED, T5/LED, some have used many fluorescents but not the best choice, and you'll end up replacing them constantly. Now lastly, I'll mention the protein skimmer. Durring the first month or so I wouldn't recomend turning it on as ecentual bacteria is developing and a protein skimmer is just going to keep removing well your dosing. This gives you some time to wait for a sale to arise or to research the one that best suits your needs. I also recommend wait till Christmas (I know it's a ways away) this will allow you to obtain some materials gradually like sand and rock (which I'll mention in a minute) then come Christmas, and you can score some wicked deals. Now rock. Most people will by rock live. Live rock is however (at least where I live) is $10 per pound. You need about a pound per gallon (I'll let you do the conversion. A gallon is 3.8L). so a way to bypass the high cost is to buy base rock (dry rock). The is non cultured dry rock which you then seed by adding one small peice of live rock and waiting 4-6 months. That being said I did this and added coral in the second month. You don't have to wait for the rock to be fully cultured. Also the more live rock you add, the quicker it seeds, but it's not nessesary. I'll check back soon and see if you have any more questions or cost ranges for things. Sorry for the long post. Just wish someone told me this when I started.