As long as the filter is well established you should be good. When I redid my 30 gallon I didn't use any old water, but adding the established water is always a plus. Just perform a couple of water tests and you'll be good to go. You'd be surprised at how fast the parameters go back to normal when using established media and with the bonus of 40 gallons it'll be even faster. It only took my tank about 24 hours to return to near normal conditions.
When adding the soil just make sure you cap it properly, meaning you add enough substrate on top of the dirt that it won't leak out into the water column. My only mistake with this is that I didn't use a deep enough cap and somehow my bumblebee catfish dug deep enough to expose the dirt. No harm was done other than me trying to figure out the reason for an algea bloom, but I eventually figured it out.
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When adding the soil just make sure you cap it properly, meaning you add enough substrate on top of the dirt that it won't leak out into the water column. My only mistake with this is that I didn't use a deep enough cap and somehow my bumblebee catfish dug deep enough to expose the dirt. No harm was done other than me trying to figure out the reason for an algea bloom, but I eventually figured it out.
Sent from my SCH-I535 using MonsterAquariaNetwork App
