Ah, yes. Let us return to Alexios and his economic reforms.

Where did the specie for his new coins come from?

According to Cécile Morrisson's study titled Byzantine Money: Production and Circulation, which can be found in the fascinating collection of essays known simply as The Economic History of Byzantium, it is stated that the reason that Alexios and his advisors chose the standard 85% purity for the hyperpyron, was so that the existing debased coinage could be melted down, refined into new hyperpyra, and introduced back into circulation with the smallest possible loss in precious metal and production time.

Refining was a lengthy process - the higher the purity, the longer it took to fashion the coins. Since Byzantium desperately needed a coinage reform by the 1090s (Byzantines were embarrassed to be using grey "mystery metal" coins in place of the old 97% gold nomisma used less than a century earlier), Alexios naturally wanted to recirculate the new coins as quickly and as easily as possible.

Specie from recycled coins and donated Church valuables was used, but was that enough?

It seems to have been. While further refinements to the coinage system lasted until AD 1109 (the reform began in AD 1092), Alexios' changes seem to have been widely accepted and used both within and outside of the Empire, and remained so for about 200 years afterwards.

However, while the hyperpyra seem to have been minted in sufficient number, the lesser denominations, such as the aspron (25% gold) and billon (7% silver) trachy, and the bronze nummoi, the coins used for everyday transactions, were almost always in short supply, leading Alexios' son John II, to debase some of them (namely the aspron trachy) to increase supply. In the end, this seems to have worked, at least until the accession of the Angeloi.

Did Alexios have to implement additional taxes?

Many of Alexios' reforms with regards to taxes were actually reinstatements of older systems that had long been phased out of use. For example, two major changes to the tax system that occurred during his reign were the epibole (roughly meaning "dividing cells") and the praktikon ("practical things").

The epibole was similar to several very old Byzantine taxation systems whereby the total value on a taxation unit (such as a district of a town, or a small community) was summed up, regardless of the tax exemptions that were allowed for various reasons. Essentially, the total value of the community. Then, when the assessment was complete, the tax assessor would then check to see if any new land cultivations or additions had been made to the unit, to which he would then add on additional tax based on the size of the new plots. Then the total tax value of the unit was divided by the number of modioi (a unit of measurement, based on crop yields which corresponded to a single nomisma under the old system) and the result became the tax indicator, which could then be used as a metric to tax each individual's holding within a community in what was officially considered a "fair" way.

The praktikon was essentially a new system completed by the time of Alexios by which owners of multiple estates or plots could consolidate their taxable holdings into a single artificial plot during the tax assessment. This eliminated a lot of the headaches that accompanied taxes assessed on land holdings in multiple communities spread across the Empire. A list of the land holders and their taxable holdings was kept in a grand index called a thesis.

However, despite these sensible reforms, Alexios was constantly derided for his decision to melt down Church decorum and other decorations during the time of economic crisis in the early years of his reign, and so often received a lot of flak for any reform he tried to push. These unpopular decisions haunted his later years, and perhaps prevented him from completely sorting out the economic issues within the Empire. But, even despite this, for what he accomplished in his life, he had done plenty to get the Empire's economy back on its feet after the financial crises of the 1070s and 1080s.