From the data provided, I would offer some observations.
I agree with 1077 on the UG, the substrate is the filter media/bed so everything that is pulled into a tank's filter in this case remains in the tank out of sight. One reason I don't recommend UG filters. I used to have them, even on my 90g once upon a time, and weekly I would push the gravel vacuum hose (minus the large end piece obviously) down the uplift tubes to the plate level and managed to suck out quite a bit of debris though not everything. The beauty of canisters is that you can easily rinse this stuff out.
Nitrates at 20-40 is frankly much lower than I was expecting to read, and this is probably due to the recent water changes that have reduced them. It is still better to have nitrate below 20ppm. You mention weekly water changes of 66% lately which leads me to assume they were not so frequent previously, and that certainly would contribute to this. You have largish fish that have more impact on the biological system so more is required in terms of maintenance. Without plants, a regular weekly partial water change of 50% is the minimum in my view. I do this in my tanks, even though they are well planted; but I have relatively heavy stocking, though small fish. Once you get this present bout under control, following this schedule will help keep it away.
And the light needs reducing in intensity, and possibly duration. Without plants, light is not necessary except to view the aquarium. Fish will be perfectly happy without lights. But we have our tanks to observe our fish, so you want a light. Definitely only use one tube, and not the actinic. I would recommend a natural daylight type, with a kelvin around 6500K, for best colour rendition of the fish. You can have it on a timer for your normal viewing schedule. While plants are better under a regular schedule, fish appreciate this too; in their habitats they have 10 hours of light and 10 hours of total darkness every day of the year, and they will be more relaxed with a regular schedule. Ten hours with only one tube might be OK, or you could always reduce it to 8 hours.
With silver dollars plants won't be too successful, but there are a few that fish generally avoid. Java Fern
is one; if you have wood or rock you can attach this plant and it will root itself on the object and add some dark green. One tube for 8-10 hours will be sufficient light so no issues there. Another that may work is Anubias
, they are tough plants (thick waxy leaves), and also attach to rock and wood. Someone in another thread mentioned a stem plant that usually survives SD's, sorry I can't remember which one, but someone else might mention it again. Any plants you can get growing will help toward keeping nitrates under control and they make use of the organics that will accumulate. The UG filter has no bearing on plants I've mentioned, so if you leave it that is not an issue.
I would reduce feedings from twice to once daily. Some aquarists have one or two days a week they don't feed at all. Fish that are mature don't need more than one feeding, and healthy fish can go days without. They always act hungry, that is their instinct, since they eat food when they find it.
The above should help resolve the green water. My last comment concerns the fish species. You have several that would be better in groups. All characins are shoaling fish, so the silver dollars, silvertips and white tetras (whatever they are
) should be in groups of minimum 5 (SD's) or more. The angels are also shoaling fish, and unless these are a mated pair, I would recommend five or more. Now, obviously if you did all this it would be too many fish in a 70g. So the answer here is to consider re-homing some. Fish will always be less stressed when maintained in comparable situations to what nature has programmed into them. But, having said that, in your case with larger fish it may be best to leave them. Though I would definitely increase the two small tetra species.
Hope this is of some help.