Indian Almond Leaves- How Much Do They Affect PH By?

The effect is greater the lower the KH. But even so it is not by much. I think you said you have soft water? If you do, your KH will also be low so it would affect your tank more than someone with very hard water.

Try them, but don't use many at the beginning. Maybe half a leaf to start with. Monitor your pH and if it stays stable, you could add more. If it affects the pH, you can always take it back out.

As an update, the Indian almond leaf didn't seem to have any effect on PH or KH according to the test strips - although a separate post elsewhere focuses on my experimentation with liquid-based equivalent tests .v. test strips generally.

The leaf was removed from the tank last week as it was decomposing.  I hadn't got round to replacing it but, goodness, activity in the tank this morning has been at hyperactive levels - I've been working at home and it's just beyond the laptop and so in my peripheral vision, leaving me feeling almost seasick at times.  It's been not dissimilar to the "swimming up and down the glass" antics that often occur in the early establishment of a new tank.  I have no idea whether it's simply chasing/fighting or mating but there's been racing round the tank, a flurry of activity among the plants, conjoined swimming in 2s and 3s, duels between the tetras, duels between the largest tetra and the largest harlequin (for a change, instigated by the harlequin), 4 tetras seemingly mating in one leaf (!!!), etc. 

All seems to have calmed at last but it may well be time to put another Indian almond leaf in for what seems to have been a calming effect when they've actually been shoaling together in a group of 12.

The new Indian almond leaf hasn't helped much / made much difference to the behaviour - or, at least, the hyperactive behaviour has been going on for 4 hours now, with the 2 smallest tetras duelling for a couple of hours and the "4 tetras conjoined in one leaf" behaviour going on over an hour this morning.  Worthy of an Attenborough documentary indeed, Extreme One/Simon!