I'm in the exact same boat as you. I'm about to setup a low light tank with no added CO2-- and try two small goldfish. Are you using canister filtration or hang on tank filtration of any kind? My tank is only 56 gallons, so I worry about the nitrates in mine. So you have one goldfish in a 75 gallon tank? Maybe I shouldn't try two in mine.
My tank is:
56 gallon (tall column with a small planting surface)
24 inches tall from bottom to top
18 inches "deep" from front to back
30 inches wide from side to side
Right now I have no water in it, but I have 1.5 inches of flourite and 1.5 inches of sand. If I am going to make a change in this plan, now is the time to do it. I bought the sand to cap the Flourite because I didn't want the goldfish to eat the flourite, but I can tell already it is impossible to keep the flourite off the surface. I am either going to a.) remove as much sand as I can and buy more flourite and cap it with pea gravel, or b.) mix the sand and flourite and cap it with pea gravel to prevent the goldfish from getting to the flourite as much as I can.
The local water company reports: "Hardness levels leaving our water treatment plants in 2010 ranged from 52 (soft) to 450 ppm (very hard) or 3.0 to 26.3 grains per gallon. The average leaving our plants is 197 ppm (very hard) or 11.5 grains per gallon. The sodium level is approximately 20 ppm. Water within our distribution system averages 7.3 pH units. Alkalinity is the capacity of water to neutralize acids; water within our distribution system averages 77 ppm. The highest Nitrate was 0.29ppm and the highest Nitrite reading was 0.017ppm."
I have tried to learn what goldfish tend to eat and what they don't tend to eat, so I want to start with a diverse assortment--
1.) Already under my lights:
Right now I have a holding tank with some java ferns (microsorium pteropus), some anubias (nana), some wisteria (hygrophila difformis), and some anacharis (egeria densa). The elodea and on bunch of java ferns came from a LFS and there are black dots on most of the leaves of the java fern that came from the LFS. The Anubias, Wisteria, and another bunch of Java Fern was that Top Fin kind from PetSmart and no black dots, though the wisteria was in kind of bad shape.
2.) Already purchased:
Tomorrow, two people here are shipping me some batches of Marsilea.
3.) To be purchased within the week:
3x Coffee Anubias (Anubias Barteri Coffeefolia)
1x African Water Fern (Bolbitis heudelotii)
1x Rotala (Rotundifolia)
2x Crypt (Parva)
1x Crypt (Nurii)
1x Crypt (Hudoroi)
1x Crypt (red Beckettii)
We're not using any added CO2, or anything like that. I do have two HOBs, an Emperor 400 and an AquaClear 50. I have a bubbler and a heater, but not much else.
If I am gonna do something better with substrate, now is the time to do that.
Can I just shove Osmocote Plus Gelcaps into my substrate every six months?