Question: Plant fertilizer and HOB water flow reduction

I have a few more questions about my tank setup and my axies.

First, I have 2 (approx. 7-8" each) juvenile axies (hatched Dec. 2016) in a 40 gal tall tank. Sand substrate, low light live plants, driftwood with spaces to hide, HOB filter with Purity in addition to reg carbon filter. Tried a feeding dish, but they insist on being hand fed, and have good appetites (each eats 2-3 red wigglers daily). All water parameters are checked daily, and are fine. I use Prime (dechlorinator/ammonia detoxifier), Stability (beneficial bacteria as the tank hasn't been running 6-8 weeks yet), Stress Coat (haven't used), and Flourish Excel (plant fertilizer).

1. I read on a old forum post on here that plant fertilizers are toxic to axies. I had assumed if it is safe for shrimp and other inverts, it would be fine for axies. I have added one dose of Flourish Excel to my tank on the day I planted it, before the axies were added. Do I need to do a huge water change and/or stop using it?

2. Is a bubble wand a good addition for aeration? I had thought to leave it out because of axies liking calm water.

3. What is a good choice for reducing water flow/current from the HOB filter? This new one seems to have a stronger current than others I've used in the past.

4. Both axies seem to always have their gills curled somewhat forward, even before when they were each in their smaller individual tanks with no filter. Other than the water flow, what could be causing this?

Thanks in advance for any help!

Update:

Question 3 is eliminated as the HOB filter I was using (and just bought new) quit on me. I replaced it with a Fluval internal filter that has a built-in spray bar and that made a huge difference.

I also dropped the water level significantly as I was told (by a local reptile/amphibian/tarantula specialty store owner/breeder) that they hate deep water and it worried me. He was surprised they had no issues swimming up to the top and back all the time! I do have tall driftwood pieces and hornwort that they love to hang out on, but I dropped the water to just below the top of the driftwood piece to make myself feel better.
So, my new question is: how deep should their water ideally be??