Celcuis

A video of their movement might help, but I would assume they are Ostracods and are just free fish food

Have to agree. The golds are even worse---hence you don't see a lot of either and they are not cheap.

These fish are very difficult to keep alive much less successfully raise fry from.  They require optimum tank conditions - hence the price. 

I know this sounds negative but have been there and done that too.  Often once you start to supply a retailer your wings are clipped so you can not sell to the general public and in the cases of more difficult to breed/grow out fish you end up with a glut that the store doesn't want due to low demand in the stores at the time.  It really is a catch 22 situation.  If you want to build a relationship with the stores you need to do it months and months before you approach the 'would you be interested question'.  They are more likely to approach you themselves if they see what you have available is top quality and then they will push for the absolute cheapest.  The bread and butter fish are cheap as chips to purchase from the main wholesalers and the likes of Animates now have their own import facilities.

^^^ What Adrienne said. You have to give them a good reason to buy from you instead of the importers. Two main options: sell at such a low price that they can increase profits more (obviously this means you don't get much), or, provide stock that has a difference, e.g. a hard to find/new colour variant etc.

If I was to suggest 'worthwhile' species that may help you cover some costs they would probably be: Angelfish (specific strain), some easy to breed tetra, Electric yellow African cichlid, maybe a nice swordtail. 

It is a bit of a "catch 22". If you sell "at the back door" (which includes trademe) retailers will not buy off you unless they are so cheap and so good that they cannot resist. At that stage there is not much in it for you.

If you are going to do corals you will need to establish a good reputation within the smallish, dedicated,  group of marine keepers throughout the country.  From experience there are no fishkeepers as likely to kick up a stink as marine owners (note - I am one of them).  There are already at several people with great reputations who frag and sell through the facebook group and trademe, who sell at good prices compared to the shops so do your homework
In the fishkeeping community, reputation is everything.
NB tempered glass I believe can not be cut.

First thing to learn is how to get cheap glass from the glaziers and how to cut glass and make your own tanks.

In a previous life I used to breed 50000 tropical fish a year which were mainly sold to one wholesaler who had 125 outlets to sell to. I also used to grow and sell aquarium plants to most of the pet shops in Christchurch as well as on selling plants from other growers. Hamilton may be too small of a market if you wish to get serious. You may need to extend your market. You do need to grow mainly bread and butter lines to pay the bills and then branch out in to some of the more risky but financially rewarding lines. If you are lucky you may cover your costs but you will not retire on the proceeds. Also made aquariums for a few years. It can be fun but best not to give up the day job.

It also takes a while to develop a reputation so that retailers or wholesalers will buy from you. You have to be able to deliver the plants or fish that they want at the time they want them.

It all depends on species of plant or animal you are breeding, the easy ones soon flood a market so need to find ones that are harder but in demand

One 625ml pack should do the tank & probably the background.  You only need to do the inside of the tank if the outside is painted with an enamel paint.
Materials required for tank:
Ply, pva glue, stainless screws, epoxy, epoxy colouring (if required), putty to fill in screw heads, paint, silicon, glass for viewing panel.
Tools:
Circular saw, jig saw, drill / screw driver, sand paper, plastic sqeegie, router (optional), paint brush.
Materials for 3d background:
Polystyrene, toothpicks, grout, epoxy to seal, silicon to glue to tank.
Tools:
Fine saw, sharp craft knife, glue gun, small sureform, sand paper, paint brush for dapping on grout, paint brush for epoxy coating. 

No to the marine ply - you don't need it.  Standard untreated ply will do just as well if coated in epoxy etc after you made the tank so long as its not the very cheap stuff that is fill of voids.  Marine ply is a lot more expensive and would still need coating.

9mm or preferably 12mm ply will make an easier build then thinner as its thick enough to be able to screw into the ends.  This thickness will also not need any external support to keep it from bowing, but I would still use a eurobrace on the top to avoid this just in case. Don't bother with marine ply as it will be coated with epoxy (my preference) or pond liner anyway.
The price of a 50 gal in ply will work out to cost more to make then the same in glass.  Ply, viewing glass, glue, s/s screws, epoxy (or pond liner), paint or stain for the outside, tools if you haven't got them ......  All adds up.  When you get over the 100 gal sized tanks then ply will work out cheaper because of the thickness of the glass needed for a glass tank.  But its a great project!
3D background.  Make out of polystyrene, a polystyrene insulation sheet from the hardware store is what I used.  Cut one piece to the size of the rear of the tank and use the rest to cut out humps, bumps, rock shapes - whatever you want.  Hold it all together with toothpicks to work out the general design you want then glue (I just used a hot glue gun).  Carve out the poly (a small sureform & 40g sand paper works well) to create the final look then use sand paper to smooth off to a water worn look if required. This process is messy and poly beads / dust will be all over the place! Coat with grout.  Use different shades of grout for highlights and shadows if you want to get artistic.  Once hardened the grout will change the PH of the water so either soak it for a month or coat with epoxy.
Those 10w flood lights should be fine for your chosen plants - maybe even a bit strong?  Ira look here to see what Celcuis is referring to: https://www.youtube.com/watch?v=LU_y6OK2qK0
I think a corner mattenfilter should work so long as it is large enough, but by design it will take up some of the room in the tank.

9mm or preferably 12mm ply will make an easier build then thinner as its thick enough to be able to screw into the ends.  This thickness will also not need any external support to keep it from bowing, but I would still use a eurobrace on the top to avoid this just in case. Don't bother with marine ply as it will be coated with epoxy (my preference) or pond liner anyway.
The price of a 50 gal in ply will work out to cost more to make then the same in glass.  Ply, viewing glass, glue, s/s screws, epoxy (or pond liner), paint or stain for the outside, tools if you haven't got them ......  All adds up.  When you get over the 100 gal sized tanks then ply will work out cheaper because of the thickness of the glass needed for a glass tank.  But its a great project!
3D background.  Make out of polystyrene, a polystyrene insulation sheet from the hardware store is what I used.  Cut one piece to the size of the rear of the tank and use the rest to cut out humps, bumps, rock shapes - whatever you want.  Hold it all together with toothpicks to work out the general design you want then glue (I just used a hot glue gun).  Carve out the poly (a small sureform & 40g sand paper works well) to create the final look then use sand paper to smooth off to a water worn look if required. This process is messy and poly beads / dust will be all over the place! Coat with grout.  Use different shades of grout for highlights and shadows if you want to get artistic.  Once hardened the grout will change the PH of the water so either soak it for a month or coat with epoxy.
Those 10w flood lights should be fine for your chosen plants - maybe even a bit strong?  Ira look here to see what Celcuis is referring to: https://www.youtube.com/watch?v=LU_y6OK2qK0
I think a corner mattenfilter should work so long as it is large enough, but by design it will take up some of the room in the tank.

Is there a reason you want to use plywood rather than glass?  That small size glass is going to be pretty inexpensive anyway.  And the whole tanks go for pretty cheap.
Cheapest lighting would be something like a 4 foot fluoro fitting, or even a couple small clip on lamps.  Which 10w floodlights are you talking about?
Better to go with a good canister filter than a sponge filter.

Ah, those ones.  I got the impression from some other threads that those were pretty poor color rendering and not a good spectrum for plants.  But haven't looked too closely into it and there is a metric buttload of various ones.

Actually isn't 15 gallons more like 55 - 56 litres? That sounds like a really, really BIG bucket ! (about 5 regular buckets?).
Ramshorn Snails do eat detritus & algae, maybe they would help?

If you are planting plants that have been grown submersed they may die off because the leaf structure is different to emersed growth. You need to grow them in shallow water in order to produce emersed leaves. They do not need to be moist, only the roots do.

I believe you will need more than this as humidity will be required.  Try messaging Liam or Camtang as they do/have done emmersed.

Filter is the correct size as commonly work on a flow of 5 times the tank volume per hour (15g tank x 5 = 75g / hr).  In this case it sounds like it was simply overloaded (over feeding & lack of water changes) & thus it looked like it wasn't working.
A filter with sponge and floss in it will be working 2 ways: Biologically and mechanically.  It will be mechanically filtering out any largish particles in the water by trapping them and the surface area inside the floss & sponge would enable bacteria to grow to turn harmful ammonia into nitrites and eventually less harmful nitrates.  What you want to do for the filter to be efficient is once a week wash the floss & sponge in siphoned off tank water (not tap water especially if chlorinated!).  This will unclog the pores of the media by removing any mulm which consists of large particles and dead bacteria & allow more "real estate" for healthy bacteria to grow.  You want the healthy bacteria to survive the cleaning process so don't use any chemicals or dry it out at all.
Water changes are still a must even if the filter is working at peak efficiency.  You can do either 10% per day or 35% twice a week or 50% per week (easiest option).  By cutting down on the feeding and doing the above your tank water should be crystal clean.
 

1. the stocking is fine with the current tank & pump size.
2. Air pump won't be needed.  It would have helped because the bacteria bloom had depleted the oxygen level, I wouldn't worry about getting another once the tank is up and running properly. You could get another filter if you want but the current one is fine.  
3. I have a piece of drift wood that has been in the tank for 2 years and still insists on floating.  You could weigh it down with a rock or glue it to a tile (hot glue gun) and cover the tile with substrate to hide it.

I wouldn't add biological supplements. Your problem has been biological overload from too many bacteria. Your biological load from that small number of fish will be low and will be the actual load from your actual fish. This will then culture the actual bacteria your tank requires (so long as you give up overfeeding).

What size and brand is your 'small' filter?  You might be surprised as sometimes small filters are pretty powerful.