I created a tax event of $15,000 by exchanging an alt into Ethereum (now Ethereum is dumping)

No hate please. Let's be civil.

But I made a rookie mistake. I made $15,000 from an alt coin that I knew was going to dump so I quickly exchanged it into Ethereum back when it was 4.1k.

I still believe Ethereum is a safe asset and I am more than happy to HODL long term. But I guess the dilemma here specifically is the short term — in Australia tax time is July which is only a month away. And I've just created a tax event of $15,000.

But now that Ethereum has depreciated by more than 60% so it would probably be around $7,000.

Normally this wouldn't bother me as I still believe the fundamentals are there long term.

But do I still owe tax on that $15,000 even though it is more like $7,000 now? And if so can I claim capital gains losses?