Awesome. So many people assume they can do what they want to native wildlife without repercussion. I do so love a good looking Native Species Aquarium though.
As far as tank setup, a 120 would be great, but buy as large as you can afford or as size allows. It is unlikely that you will regret a bigger tank. I've sold many tanks, and it's not uncommon for me to have the customer comes back days later to say that it looks smaller in their house.
As far as filtration is concerned. This is a matter of opinion. There are many various options for filtering an Aquarium. If you ask me I will say; For "Hang on the Back" filters I would suggest TWO of the Aqua Clear 110 units. For canister filters, I personally like the Fluval FX5 Filter, which is rated for 400 Gallons. And lastly, Sumps or Wet/Dry filters are effective in their own right.
Do you have a filter on the 30 gallon now? When you upgrade, you should put whatever filter media or cartridge that is in that filter, into the bigger filter with the new unused media.This will hopefully allow the establish bacteria colony to spread into the new sterile media. During this time I would suggest using a product per labeled instructions, called Stabilize made by SEACHEM. If this product is not available to you, try NurtaFin CYCLE- though Seachem has a great reputation for quality and Nutrafin is very run-of-the-mill. These products help to stimulate growth of the "Waste Eating" bacteria. Remember to use a product like SEACHEM Prime, or Nutrafin AquaPlus to remove Chlorine, Chloramine, etc from the new water you fill the new tank with.
Alternatively, you could keep the LMB in the 30 gallons for 6 weeks after the new tank has been up and running. Once the tank has been filled for a couple days, and the filter in running and the temperature has stabilised you could add fish to preform your cycle. A common fish to use are Danios, Such as Zebra Danios. They are very hardy fish and are better adapted to survive in water with Ammonia and Nitrite, which is going to spike during the 6 week period. These fish can cost between 0.75-2.99 each. If you can get them for about a dolllar each, I would add 3 dozen or so. These fish will produce Ammonia, which feeds the bacteria, which converts it Nitrite, and another bacteria eats at and converts it to Nitrate. Once there is enough bacteria in the tank, you wont see Ammonia and Nitrite, but only Nitrate. At this point you could remove the Danios (or leave them to be eaten) and put in your LMB. During the period with the Danios only, using the previously mentioned products will help the cycling process.
Lastly, you can prepare the new aquarium with what is called a "Fishless Cycle" which involves adding Ammonia daily into an aquarium. There is plenty of info on this if Googled.
Live plants are not required at all. They are simply one means of removing Nitrate from the tank. Since the LMB likesto play with them and redecorate the tank on his own accord, IF you wanted to use plants in your aquarium, you might try sticking live bamboo down the back slot opening of the tank, or by floating a common vineing philodendron houseplant called Pothos. The leaves will grow out of the water, with the roots hanging down from the surface. Enough plants in the aquarium and you might find that you need to do waterchanges every 2 weeks, rather than weekly. After a tank is cycled, Nitrate testing is what will tell you how frequent you must do water changes.
The only way the plants could make the aquarium cloudy was if they were rotting. Could be coincidence that the plants were in at the same time you had a Biological boom which makes the water cloudy too.
Algae is a natural- non harmful plant that goes because the tank has certain nutrients that are not being used. Most commonly these are Nitrates, Phosphates, and ofcourse Light. If your water params are in check, and algae grows there is no problem. It is just considered unsightly, yet it is found everywhere in the wild in various forms. If you want the tank to truely look like a slice of the river/lake you collected from, I would let it grow on the rocks or wood, and simply scrape it off the viewing glass. Snails are great to keep the algae growth in check. Reducing the light that the aquarium receives will do the same.
Also, the more liveplants you have in an aquarium, the less algae grows. The plants use the same nutrients that algae does.
Finally- you better post some photos! MFK loves photos.