Who Gives A Fish Forums

My discus fry aren't growing like they should, and I think it's the water hardness. I've got a 40-gallon tank, 0 ammonia, 0 nitrite, 5 nitrate, pH 7.2, but the GH is 12. What's the best way to soften the water without messing with the pH too much? I'm using RO water mixed with tap, but it's still too hard. Filter is a Fluval 307, substrate is sand, and I've got some driftwood in there. Feeding them beef heart mix twice a day.

If your GH is 12, that's definitely on the high side for fry. RO water should help, but you might need to adjust the ratio. Try 75% RO, 25% tap and see if that cuts it. Also, check your TDS—aim for under 100 ppm. I've had success with peat moss in the filter to soften water naturally. What's your current TDS reading?

blueguy wrote: ↑Fri Aug 15, 2025 5:41 pm If your GH is 12, that's definitely on the high side for fry. RO water should help, but you might need to adjust the ratio. Try 75% RO, 25% tap and see if that cuts it. Also, check your TDS—aim for under 100 ppm. I've had success with peat moss in the filter to soften water naturally. What's your current TDS reading?

Peat moss is a solid idea. I'd also recommend adding almond leaves—they release tannins that soften water and give the fry a more natural environment. How often are you doing water changes?

I've been in the same boat with my fry. Water changes are key—I do 50% daily for the first month, then taper off. My GH was 10, and I got it down to 6 by using straight RO with a pinch of mineral supplement. Fry started growing way faster after that. What's your temp at? Mine are at 86°F, which seems to help their metabolism.

Yep, high GH can stunt growth big time. I'd ditch the tap water altogether and go full RO with a remineralizer like Seachem Equilibrium. That way you've got total control. Also, make sure your beef heart mix has enough vitamins—weak fry can't handle hard water as well. How old are they?

Boldy wrote: ↑Fri Aug 15, 2025 8:57 pm Yep, high GH can stunt growth big time. I'd ditch the tap water altogether and go full RO with a remineralizer like Seachem Equilibrium. That way you've got total control. Also, make sure your beef heart mix has enough vitamins—weak fry can't handle hard water as well. How old are they?

Full RO is the way to go, but watch the pH swings. If you're not careful, it can crash. I buffer mine with a bit of crushed coral in the filter. What's your KH at? That'll tell you how stable your pH is.

Might be worth checking your tap water's GH too. Some areas just have crazy hard water. If that's the case, RO is your only real option. I've seen fry bounce back fast once the water's right. Are they eating well, or kinda picking at the food?

Thanks for the advice, everyone. I'll try the 75% RO mix first and grab some peat moss. My TDS is 150, so I've got work to do. Fry are about 3 weeks old, temp is 84°F, and KH is 4. They eat like crazy, but growth is slow. I'll update in a week.

Good call on the peat moss. Just don't overdo it—start with a small amount and monitor the GH daily. And bump that temp to 86°F if you can. Every degree helps with fry.

blueguy wrote: ↑Sat Aug 16, 2025 1:40 am Good call on the peat moss. Just don't overdo it—start with a small amount and monitor the GH daily. And bump that temp to 86°F if you can. Every degree helps with fry.

Yeah, temp makes a huge difference. I've had fry double in size just from a 2°F increase. Also, keep the lights low—less stress, more eating.