First, and please don't take this as rudeness, please no more fish right now. I know I did say clown loaches need a group. In this case however we're talking about 6-10 fish capable of reaching a foot in length, enjoy actively swimming, and require pretty pristine water. Ideally clown loaches shouldn't be kept in any aquarium less than 180 gallons. The 75 gallon is bigger, but it's still nowhere near big enough.
To test pH it is best to fill a container and test it after 24-48 hours. Some people chose to use a bubbler. The reason for the wait is to allow CO2 to off gas and that will give you a more accurate reading, since CO2 is acidic and will off gas in an aquarium on it's own.
Again, I can't speak for test strips, but it seems they are giving you difficulty with reading them. A test isn't much good if you can't figure out what it's telling you. You might wish to instead get a liquid test kit.
You have algae eaters in the 29 gallon too? Do they look like
this? If so those are Chinese algae eaters and they prefer to not be with each other due to being aggressive and territorial. If there are that moving one to the 75 gallon might not be a good idea either since the redtailed black shark is also territorial and definitely doesn't like species like Chinese algae eaters. The 29 is too small as well if I guessed the right species. I'll let you read on your own about the attacking other fish.
Does the catfish have nasal barbels right in front of its eyes? I'm trying to figure out if it's a small, peaceful species or a larger one that is quite possibly capable of eating your tetras.
More bad news. A koi is only suitable for a pond. They aren't aquarium fish at all.
The light tan carp looking fish I have no guess at. Well I have several, but not one that I think it may be over all the others. Any chance for a picture?
Plecostomus could refer to a number of species. I've found that when most people say just that they mean
this. I could be wrong, let me know.
Lowering the temperature would be fine for the goldfish, koi (which you should seriously consider rehoming), and even pleco (if I got the right species), however it won't be as well liked by the others. The carplike fish I can't even say.
A shrimp won't actually remove any waste. No clue what kind you have, but none actually take out nitrates from a tank.
I'll ask someone with saltwater experience to come help, or else you can post in that section. Sorry, but I'm not very familiar with those setups. The use of nitrifying bacteria though doesn't make sense to me. Unless things with saltwater are completely different. Those types of bacteria are added to cycle an aquarium. Nitrates are the end result of the ammonia cycle (well in aquariums at least) and require water changes to remove. In other words if things are the same as in freshwater the bacteria you were told to add will do nothing for you.
I feel really bad about giving you what amounts to a ton of bad news.