I did fish-in cycling for years and never had a problem. Got away from the hobby for 20 years and just set up a tank 3 and a half weeks ago. 40 gallon with a fluval c3 with some matrix in the carbon chamber. 4 platys on day one, light feelings, 2 more females on day four because one male was terrorizing it's look-alike male. Light feedings for a week (what they could eat in 30-45 seconds 3 times a day). I did pick up a bottled bacteria product at that time to supplement with since my original plan was to wait two weeks between fish additions. Added some that night and again 2 nights later. Zip for ammonia or nitrites or nitrates. 50% wc then up the feeding, more feedings every day, like 6-8. Testing water every other day. All parameters 0, except ph which has held a steady 7.6 from the start. After 2 weeks, a 50 % wc then added more fish than I planned (long drive).
Day 14 added 8 rasbora hets, 9 rummy nose, 3 albino corys, 2 otocinclus. Fed light for three days, 3 meals at 30-45 seconds each. Added another dose of the bacteria product (nite-out) the first night just in case. Test every day, still nothing. Start feeding 6-8 times a day, same 30-45 seconds a feeding. Testing daily at this point, still good but barely notice a hue on the ammonia test a couple times after multiple feedings. Still no nitrite but nitrate barely starting to show. Day 18 about a 30% wc, day 22 about a 60% wc.
A couple of things. In the past I had always robbed some gravel from an established tank. This time I picked up an Anubias on driftwood and a couple of potted swords at the addition of the first fish. I was hoping the driftwood would be a good seed source, seems to have been. I added a banana plant and a java fern mat the day I added the large group of fish.
Large weekly wc's are largely a product of trying to remove tannins put off by mopani wood I had boiled for a couple hours and added to the tank for decor, but I sort of like doing them and will probably keep it up at about 50% weekly. The tannins are starting to not return as quickly.
I was in the fish business for a number of years and visited a lot of people who had problems with their new tanks, most involved what I thought was a huge amount of food laying around, often with little fuzz balls plainly visible on their decor. I was told day one in the business to treat fish as though their stomachs are roughly the size of their eyeball and if you see food floating around 60 seconds later to cut back. It's always done me well with the non-carnivores. The rasboras I bought were on the skinny side and have all fattened up nicely in just the 11 days I've had them. I actually got to my current load level a bit quicker than intended, but so far so good. I'll test again tomorrow and update if there are changes, but I really don't expect any except for maybe a bit more nitrates.
My thinking on the whole fish-in versus not is that there are several workable ways to establish a tank, and if you aren't careful, many many more ways to mess it up. Just be careful whichever method you use and you're well on your way to success.
