Try adding 3-4 live plants if you don't already have any they will help by consuming some of the nitrate.
TBH live plants at this very point of time could just make the situation worse. Shops frequently sell non-aquatic plants or high-light difficult plants and say they will be fine. Plants will only help if they are thriving, if they are dyeing or not growing they will be no help at all. A plant that is dying or going brown will be making the water WORSE.
If you want to keep your plants i suggest posting a picture for ID and post what wattage and type of lighting you have on the tank
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I can't believe no one else has mentioned this or maybe I just missed it but the problem isn't the nitrates. Nitrate is hardly toxic at 75 ppm. The problem is the .8 ppm of ammonia in water with a pH or 8.0. That is deadly as the toxicity of ammonia rises with the pH. Besides the things already mentioned, you should be doing daily water changes to reduce the ammonia and nitrite levels until they are both 0 for a full week. That will also lower the nitrate which can only be lowered with water changes. Plants will consume some nitrate but not enough to keep them low.
Nitrates may not exactly be
toxic at 75ppm, but this level can easily be stressing the fish and making the situation much worse. Under 40ppm is generally considered good.
pH 8.0 is too high really, and as stated, it is making the ammonia, nitrite and nitrate more toxic to your fish.
Aside from the fact that nitrogenous waste is more toxic at such a high pH, that pH is not the best for the fish species you keep. The high pH will could have even killed your tetras alone depending on where they came from, and although it probably wouldn't kill the angels, it could do them harm over the long term (once again to what extent depends on their origins). Was the pH always that high in the tank? If so after two years the fish will be used to it (and may have even been originally bred in a similar pH) but it will be doing them no favours, slowly lowering the pH to 6.5-6.8 will make a world of difference to the overall health of the fish.
You need to start thinking about slowly lowering your pH, but don't use chemicals. First we need to know what's causing the high pH, can you test your tap water for pH (testing for nitrate while your at it would help too).
Its also very possible the decoration in your tank is raising the pH, what decoration do you have in your tank (like rocks or shells)?
The 'Chinese algae eater' wiil most likely show aggression towards the angels a some point, after discus (although no-one in their right mind would keep them with discus) angels are their favourite fish to pester.
And IMO if your going to be keeping goldfish in an aquarium over the long term, you will need an oversized external filter
.
