The taxing system was not substantially different from the one present in Castile, with some exceptions of course. The 20% you mention is the famous "Quinto del rey" or "Quinto real", which did not apply to all wealth, it was the King's part of the mineral wealth and precious stones, as regulated by the ordinances of 1504. I translate:

We command that all the neighbours and inhabitants of our Indies that would take or take out, in any province or part thereof, gold, silver, lead, tin, quicksilver, iron, or any other metal, shall pay Us the fifth part of the net of what they should take out

In 1536 and 1540, this was made extensive to any mineral wealth or pearls taken in battle from the Indians, or through commerce. This was more of a matter of clarification than anything else, as it was abundantly clear that the rescur paid by Atahualpa was subjected to the Royal Fifth, but that Royal Fifth was taken when the gold was melted and transformed into coins by the veedores generales. That Royal Fifth, by the way, was astronomical, with Francisco López de Gómara indicating it at very close to 400,000 ducats, which is in and of itself very mind-blowing.

In 1611, however, in order to attract specialised colonists, a law was passed diminishing the royal part on iron, copper, lead, and tin to a tenth instead of a fifth for a period of ten years. This tax cut was intended to promote industry and mining those metals, as copper became more necessary for coins. With the devaluation of coins, the frequent procedure was to have the new coins contain a lower amount of silver and larger of copper.

Of course, this was not the only existing tax present in America, so let's comment on what could someone find.

- Impuesto de avería. This was a tax to be levied in America with the specific purpose of financing the Fleet of the Indies and the naval infrastructure. It could get as high as 40% of the value of the goods transported through the Fleet System. This tax was eventually dismantled, as it resulted in a lot of corruption and tax fraud.

- Almojarifazgo de las Indias. It was a tax on the commerce of the Indies. 2.5% of the value of the goods was to be paid in Seville, and 5% on arrival to America. With time, it was doubled, 5% to be paid in Seville, and 10% in the port of arrival in America.

- Derecho de palmeo. This tax was based on the space occupied by the goods in the ships. As it was paid by volume, it was intended to favour the commerce of high-value goods that did not take much space. It was the successor of the almojarifazgo.

- Extranjería. A special tax on foreign ships operating under the regime of administrative concession in the Fleet System.

These taxes made the commerce of the Indies quite expensive, with merchandises suffering taxation of 35-40% of their value, which would result in the painfully foreseeable contraband and corruption in the American ports and in Seville. But there are more and different taxes:

- Estanco. This was applied on certain types of goods, like strong spirits, tobacco, officially stamped paper, and playing cards. Salt was taxed by the estanco too.

- Alcabala. Same as in Castile, this was a tax on transactions, akin to a modern day sales tax. The alcabalas would be normally beetween 2 and 4%. Books, instruments for religious services, mules, bread, and medicines were exempted from the alcabala.

- The sisa, coming from Castile. This was a tax on meat, wine, oil, and wheat. This impositive burden was not permanent, but temporary and would be levied when there was a specific need, like reconstructing some infrastructure.

- Pontazgos, portazgos, barcajes, etc. These ones are tolls to be paid at bridges, gates, or river crossings. Their fees would vary from place to place, as they were up to the concejos or cabildos.

And last, but definitely not least, the Indian tribute. The natives were vassals to the Crown, and as such were subject to taxation straight from the Crown instead from the usual feudal lords. The contribution was communal by nature, and was to be footed by each community as a whole, with exceptions (caciques and alcaldes de indios were exempt). The tribute could be paid in pecuniary form or in goods, as famously accredited by the Codex of the Tributes (Matrícula de Tributos)