Relativley new aquarium with ick

[EvaporatingFish]

i bought a 45 gallon aquarium from petsmart and let it cycle for about 3 weeks before putting any fish in it. (its been running a Fluval Aquaclear 30-50G filter, heater set to 75F and had a gravel substrate since day one) first i added 3 twinbar platys. a week later 5 cherry barbs. then a bristlnose pleco another week later, and finally i moved my favorite (rainbow Shark) out of my girlfriends 10G tank into my 45G. All the fish have been together with no problems until 4/6. On sunday i decided to change my substrate to Sand because i think it has a more natural look to go with my decorations in the tank. so i went to petco and grabbed 2x 20LB bags of sand. Ghost shrimp caught my eye so i grabbed three of those for my tank too. I siphoned out half the water into an ice chest, put my fish in it (the first cherry barb i got in the net immediately died. brought the net out of the water and it just stopped moving. i put it back in the tank to see what it would do and it didnt move for an hour. i assumed the worst and euthanized it by throwing it in a pot of boiling water) with the filter and began changing my substrate (no extra tank for them to be in) scooped out the gravel and slowly added the sand. once it settled i put the fish and water back in the tank. about an hour after they were in it i noticed white spots on the cherry barbs. since i only had my 10G for about 2 months im still relatively new and thought nothing of it. i thought it was just sand cause the new sand was white. later that night one cherry barb died. again, still new Monday i noticed my rainbow shark rubbing on stuff but didn't think much of it. and now, Wednesday evening, i came home to the last three cherry barbs dead, one orange and black molly dead, one twin bar barely alive, and white spots on every other fish. i looked it up and immediately went out and bought API super Ick Cure. so at this point ive assumed all my fish are a lost cause. 3x twinbars, 1x orange and black molly, 1x Rainbow Shark, 1x bristlenose pleco remain.
my question is did i somehow cause this outbreak? could the substrate swap have caused it? i cleaned the sand reeeeally good before putting it in. also, is there a chance my remaining fish will make it? due to my newness i didn't see that there was an obvious problem and have already lost 6 fish.

i apologize for the length but i want to know if anything i did could have caused this and what i should do differently for future cases. if i missed any important information please let me know.

 

[Fresh2o]

Any of the fish recently purchased may have brought in Ich and the combination of the water quality in the new tank and new substrate may have induced stress and affected their immune system.
When you say you cycled the tank for 3 weeks, were any fish in the tank at that time? Did you have a source of ammonia (assuming you may have been performing a fishless cycle)? Do you own a test kit for ammonia, nitrite, and nitrate? Highly recommended for a new tank.

 

[EvaporatingFish]

all of the fish had been in there for about a month with no problems at all. i did a fishless cycle for 2 weeks. i added API bacterial supplement the very first day i set it up. at the start of week three i moved 2 black and orange mollys from my girlfriends 10G tank to my 45G tank. after 1 week i started adding fish at week increments. i have a cheapo jungle test kit that tests Nitrate (0), Nitrite (0), Hardness (hard), Alkalinity (roughly 180), and ph (between 8 and 8.4. so pretty high) but no amonia test kit. also, i know this test strip is fairly unreliable since its cheap. my fish showed no signs of stress (as far as i know) until the first cherry barb death in the net. also, one of my twinbars just died. its looking pretty grim for the little guys 2x twinbars, 1x black and orange molly, 1 rainbow shark, and 1x bristlenose pleco remain. the only new residents for the last month were the recently added ghost shrimp. but they cant get itch right? they were added just a couple minutes before the substrate swap.

also, i do weekly water changes/tank cleaning of about 15%-20%. i add water supplement directly to the bucket and let it sit for about ten minutes before adding it back to the tank and make sure its the same temp as the tank. light stays on from 6am-4 or 5pm ish

 

[Fresh2o]

The zero nitrate and unknown ammonia indicates that the tank was most likely not cycled properly. Two weeks is a short period of time with a (assuming) new filter. The effectiveness of the bacteria starter may be questionable at best, but without a source of ammonia the cycle would not actually start. Monitoring a cycle, whether fishless or not is nearly impossible without a test kit that includes ammonia. I would get the API FW Master kit (online usually a better price). The culprit could be ammonia poisoning but without a way to test it, it's hard to say. The water changes may not have been sufficient for keeping down the toxins (ammonia, nitrate, nitrate) low enough.
The shrimp may not have been carrying the Ich spores but the water they were transported in might have. The last time I bought fish from PetSmart I did not quarantine them and it resulted in Ich spreading to the main tank.
I would navigate to the top of the "Freshwater & Brackish - Getting Started " section and look for the stickies regarding cycling with fish and read up.

 

[EvaporatingFish]

i just put in the ick killer for the second time. 9 of them didnt make it. including my favorite, the rainbow shark. the remaining three seem to be getting better. the pleco really seems like it isnt effecting him at all. the orange and black molly and twinbar look pretty rough on their fins but im going to get something to help them get better. thank you for helping me. hopefully next time ill spot the signs early on and kill it quickly.