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(continued ...) If there was any sign of murkiness or turbidity and this system normally does not get above 160 F (71 C) then the murkiness could be microbiological activity. For that, the next thing to do is run a biocide through the pipes to rule out microbiological activity. That means a 5% bleach solution. Again, use your volume of liquid drained out and refill with softened water and the appropriate amount of bleach *1. Use the circulating pump to circulate the solution for about 4 to 8 hours. Drain the solution, dilute and dispose of properly. Refill the system with plain water; circulate for a few minutes and then drain. Refill and drain a second time.
Next, refill the system with the proper permanent solution. If it was clear water or murky water, then use softened water to do a refill. At your home center, you can get some water treatment to add for plain water hydronic system. The treatment should include at the very least a corrosion inhibitor and pH buffer. Follow the directions on the label. If it was glycol and you measured the percent earlier, buy new glycol and blend accordingly. Propylene glycol is more environmentally friendly the ethylene glycol. But ethylene has better performance characteristics. Use corrosion inhibited and pH buffered glycol. Not sure what to blend the glycol for? What is the coldest it will get where you live? Subtract 10 degrees and that is your glycol blend target. More glycol protects more against freezing but less glycol is more energy efficient. If you calculate less than 30% glycol, add water treatment as if it was 100% water.
Very old hydronic systems did not use a bladder expansion tank ...