I recommend using eSHa NDX as according to eSHa "All eSHa products for fresh water are suitable for use in fresh water aquaria containing fresh water shrimps." But I would still do a large water change before stocking the tank..
Parasites are unlikely if you haven't added new fish recently, and would usually involves other symptoms such as flashing against the decor and substrate for external parasites or weight loss for internal ones.
Your water is rather too hard for Otocinclus and for M.maccullochi, and right on the upper limit for the minnows so that may explain their demise.
To be honest there are no fish that can cope in 18L. With the addition of a heater it could be used for shrimps though. In some countries these small tanks aren't allowed at all, but here, the onus is on the buyer to select appropriate stock.
You'll need a heater in your minnows tank for the winter by the way, unless your house is heated 24/7.
How often are you changing water and how much each time? The recommended minimum is 25% a week and up to 50% for fish like goldfish. With regular large water changes you shouldn't need any nitrate reducing media. If you do decide to use it, Juwel sell nitrate reducing sponges for their filters, but they need regular replacement.
I don't think it's the temperature as goldfish are tolerant of quite high temperatures for short periods. The biggest risk with warmer water is that it holds less dissolved oxygen, but if that was the issue the fish would be gasping at the surface rather than sitting at the bottom.
High nitrate is a common cause of bottom-sitting, so get the nitrate test as soon as you can and see what it reads. Ideally for goldfish it should be less than 40 ppm.
I'm aware that you're the victim of very poor advice, so don't think I'm criticising you, I'm just going to state the facts.
The 105 litre tank you have in mind is not big enough for two fancies long term; they need at least a 180L tank.
Minnows and goldfish shouldn't be house together as when the goldfish grow the minnows will become food.
If you can't house a bigger tank then the best thing to do is to return the goldfish to the shop explaining they were bought by a well-meaning friend who didn't realise your tank is too small.
Get a 60 litre or bigger for the minnows and move them to it asap as whichever tank they're in they'll be in a fish-in cycle and more water volume means more dilution of toxins. Follow this guide to a fish-in cycle. When the tank is finally cycled add a few more minnows a they are a shoaling species and like a good-sized group.
I'm suspicious that this isn't outright bullying, but maybe breeding behaviour and the large fish is male, trying to get the other one to breed. This can sometimes happen even if both are male with fancy goldfish because their rounded bodies look female. If the smaller one gets too exhausted you'll need to separate them.