Brandy

columbian tetra?

This is very true of a large tank, but much less true in a nano tank--If you are floating out bark on a dry gallon or so of soil in a 5g bucket you can do it in an evening. But for a large amount,...it will take a bit.

probably because it is prone to crack, lol.

Doesn't have to be part of the rim--it still acts as a brace as @smallDollarBigHeart describes.

Ok, so two things. One, the 90 degree ledge was providing a stiffening brace and due to the crack that is gone. Two, glass can and does bow a LOT without breaking, but as your tank was designed with the brace, it is safer with the brace. So on to repair. If you can, drain the tank completely. Sister on a new bracing piece of glass to the underside of the ledge with a ton of silicone. Look up sistering a cracked 2x4 if you don't have a clear picture of what that means. The idea is to leave the cracked brace in place and strengthen it with silicone and uncracked glass. Allow it to cure the full length of time before refilling completely. If draining the tank completely is beyond you, you can possibly get away with draining 50-75%--enough to get the glass rim dry, and allow it to stop bowing until the silicone cures. If, when you refill the tank, bowing doesn't happen, you are in good shape. If it still bows, you need to think about a replacement tank.

Agree with @Colu, probably a leech. especially a "suction cup" end. see this article for pics. https://www.aquasabi.com/aquascaping-wiki_parasites_leeches

what is the basement floor made of, and is it level? Bare concrete or some kind of finished floor?

Just in general (across animal types), the albino/lutino/platinum type genes are generally recessive. This would mean that you may get all wild type body color, with fin variations in the first generation, but subsequent generations would be 25% platinum again, as the first gen offspring would all be platinum carriers. Breeding back to a platinum expressing parent would bring that to 50%.

More plant cover, patience, and a pellet of shrimp food maybe?

I think they are harmless mostly, but yucky, indeed.

hard to tell could be a leech?

Well, I totally agree with what @Mmiller2001 is trying to say here, but I think the way he has said it is possibly a little confusing. Algae is from imbalance. Too much or too little phosphate would not be balanced, but the very idea of balance means you have more than one factor to consider. 3ppm may cause an imbalance in one tank that is low on every other nutrient, or on light, and in another heavily dosed tank with fast growing plants and high light it may be fine. Your source water composition also plays a role. OVER dosing anything would cause an imbalance. The trick is we usually only have ways to measure and control a fraction of the factors at play, and so we are often guessing. I have found this video by @Irene to be good at explaining this idea.

the plants were likely grown outdoors in a pond. if you keep washing them off consistently they will go away permanently, unless you have them established in your houseplants or something.

Otocinclus and corydoras. I thought they were dull in stores...no colors, and you need so many, they take space from more colorful fish...but they are so hilariously fun to watch!! Their personalities are just charming, I could just keep them alone and be happy.

You can try just rinsing them off in the sink. If they continue to come back, you can put the water lettuce in a seperate container and rinse in mild soapy water, then rinse with clean water very well before returning to the tank.

do they look like this?

can you take a pic? could be an aphid or mite. Not bad for the fish, but probably not great for the plants.

For Otos, in my tanks, they need 10+ gallons apeice to have enough algae to eat. But you may have more algae, or plan to target feed--your mileage may vary. I have 4 in a 40 gallon, and 3 in a 29. They are stripped completely of algae and I am putting in cucmber slices and repashy...

depends on your temperature. If cooler, with some current maybe, the loach. If warmer and or stiller water, the otocinclus. However, both can be tough to feed once the algae is gone, so don't get a ton unless you are prepared to feed them.

Hmm. While I expect Candi knows more about this than I do, agree with @Kirsten, and think a test with a distilled water mixture is a good idea. I would agree that looks like you have surpassed the test range, the question is what direction....

measure the diameter and then check amazon for "black pvc"

Mine is the same as yours, and they don't make boat parts. But I get them to cut things to size and then can often assemble or get someone else to assemble them for me. They are quite good at precision cuts I have found, even 45 degree angles.

That is reallly smart @Anita! you could have basically an acrylic window, and plywood box, set the tank inside.

The material itself will be expensive but the effort would be low. I am pretty sure I could do it, not that I have the time either, lol.

Instead of trying to find a custom tank builder, I would find an acrylic shop. In my area there is a TAP plastics, and most of my boat projects start there. It would get spendy, but they could just make you a waterproof box with extra thick acrylic. I think due to the small size there are few concerns requiring specific aquarium expertise. Make a 5 sided rectangle and put a center brace in. Easy peasy, and they will either do it or know someone who can/will.